sys/newlib: Allow building default syscall implementation for mips

We can either use the UHI syscall implementation or the RIOT default
one (targeted at VFS, uart-stdio will be added once uart rx is added
to pic32).

sys/newlib_syscalls_default/syscalls.c

@@ -58,6 +58,8 @@ extern char _sheap;                 /* start of the heap */
 extern char _eheap;                 /* end of the heap */
 char *heap_top = &_sheap + 4;
 
+/* MIPS newlib crt implements _init,_fini and _exit and manages the heap */
+#ifndef __mips__
 /**
  * @brief Initialize NewLib, called by __libc_init_array() from the startup script
  */
@@ -116,6 +118,8 @@ void *_sbrk_r(struct _reent *r, ptrdiff_t incr)
     return res;
 }
 
+#endif /*__mips__*/
+
 /**
  * @brief Get the process-ID of the current thread
  *