build system: Fix linker feature test with newlib 4.3.0

Before the build system used something like

    echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -Wall -Wextra -pedantic <FLAGS_TO_TEST>

to check for flags supported by the compiler (used as linker frontend).
This however no longer works with newlib 4.3.0, which requires the
symbols `_sbrk_r`, `_close_r`, `_lseek_r`, `_read_r`, and `_write_r` to
be present. Instead, now a new file `minimal_linkable.c` was added that
adds all the missing symbols and is used in the linker feature tests
instead.

dist/tools/testprogs/README.md

@@ -0,0 +1,15 @@
+Test Programs
+=============
+
+This folder contains a collection of test programs for use by the build system,
+e.g. to check for compiler features.
+
+`minimal_linkable.c`
+--------------------
+
+This is a minimal C program that is expected to compile and link with all
+supported versions of GCC and newlib.
+
+Note: We currently even link with GCC when compilation is done with LLVM. It is
+expected to compile and link fine with LLVM as well, but may require more some
+additional flags to find the embedded C lib to link against.

dist/tools/testprogs/minimal_linkable.c

@@ -0,0 +1,55 @@
+#include <reent.h>
+
+int main(void)
+{
+    return 0;
+}
+
+void _exit(int n)
+{
+    (void)n;
+    while(1);
+}
+
+void *_sbrk_r(struct _reent *r, ptrdiff_t incr)
+{
+    (void)r;
+    (void)incr;
+    return NULL;
+}
+
+int _close_r(struct _reent *r, int fd)
+{
+    (void)r;
+    (void)fd;
+    return 0;
+}
+
+_off_t _lseek_r(struct _reent *r, int fd, _off_t off, int whence)
+{
+    (void)r;
+    (void)fd;
+    (void)off;
+    (void)whence;
+    return 0;
+}
+
+_ssize_t _read_r(struct _reent *r, int fd, void *buffer, size_t count)
+{
+    (void)r;
+    (void)fd;
+    (void)buffer;
+    (void)count;
+
+    return 0;
+}
+
+_ssize_t _write_r(struct _reent *r, int fd, const void *data, size_t count)
+{
+    (void)r;
+    (void)fd;
+    (void)data;
+    (void)count;
+
+    return 0;
+}

makefiles/cflags.inc.mk

@@ -67,7 +67,7 @@ endif
 # Check if linker supports `-z noexecstack`. Handle BUILD_IN_DOCKER separately,
 # as this is run in the host environment rather than inside the container. We
 # just hardcode this in the BUILD_IN_DOCKER case for now.
-LINKER_SUPPORTS_NOEXECSTACK ?= $(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -Wall -Wextra -pedantic -z noexecstack 2> /dev/null && echo 1 || echo 0)
+LINKER_SUPPORTS_NOEXECSTACK ?= $(shell LC_ALL=C $(LINK) $(RIOTTOOLS)/testprogs/minimal_linkable.c -o /dev/null -lc -Wall -Wextra -pedantic -z noexecstack 2> /dev/null && echo 1 || echo 0)
 
 # As we do not use nested functions or other stuff requiring trampoline code,
 # we can safely mark the stack as not executable. This avoids warnings on newer

makefiles/libc/newlib.mk

@@ -2,7 +2,7 @@ ifneq (,$(filter newlib_nano,$(USEMODULE)))
   # Test if nano.specs is available
   ifeq ($(shell $(LINK) -specs=nano.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
     USE_NEWLIB_NANO = 1
-    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
+    ifeq ($(shell LC_ALL=C $(LINK) $(RIOTTOOLS)/testprogs/minimal_linkable.c -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
         CFLAGS += -fshort-wchar
         LINKFLAGS += -Wl,--no-wchar-size-warning
     endif

makefiles/libc/picolibc.mk

@@ -2,7 +2,7 @@ ifneq (,$(filter picolibc,$(USEMODULE)))
   # Test if picolibc.specs is available
   ifeq ($(shell $(LINK) -specs=picolibc.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
     USE_PICOLIBC = 1
-    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -specs=picolibc.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
+    ifeq (LC_ALL=C $(LINK) $(RIOTTOOLS)/testprogs/minimal_linkable.c -o /dev/null -lc -specs=picolibc.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
         CFLAGS += -fshort-wchar
         LINKFLAGS += -Wl,--no-wchar-size-warning
     endif
@@ -31,7 +31,7 @@ ifeq (1,$(USE_PICOLIBC))
   # picolibc. But since (as of now) RIOT only supports disabling the execute of
   # all RAM via the `mpu_noexec_ram` module, permissions of the segments are
   # ignored anyway. So for now, we just simply disable the warning.
-  LINKER_SUPPORTS_NOWARNRWX ?= $(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -Wall -Wextra -pedantic -Wl,--no-warn-rwx-segments 2> /dev/null && echo 1 || echo 0)
+  LINKER_SUPPORTS_NOWARNRWX ?= $(shell LC_ALL=C $(LINK) $(RIOTTOOLS)/testprogs/minimal_linkable.c -o /dev/null -lc -Wall -Wextra -pedantic -Wl,--no-warn-rwx-segments 2> /dev/null && echo 1 || echo 0)
   ifeq (1,$(LINKER_SUPPORTS_NOWARNRWX))
     LINKFLAGS += -Wl,--no-warn-rwx-segments
   endif