cpu/cortexm_common: add XFA handling to linkerscript

The global core/ldscripts/xfa.ld doesn't match our cortexm_base.ld.
This commit directly adds the two XFA lines to cortexm_base.ld.
In addition to that, a dummy (empty) xfa.ld is added, which the linker will pick
instead of core/ldscripts/xfa.ld, effectingly not using it.

cpu/cortexm_common/ldscripts/cortexm_base.ld

@@ -55,6 +55,7 @@ SECTIONS
         *(.text .text.* .gnu.linkonce.t.*)
         *(.glue_7t) *(.glue_7)
         *(.rodata .rodata* .gnu.linkonce.r.*)
+        KEEP (*(SORT(.roxfa.*)))
         *(.ARM.extab* .gnu.linkonce.armextab.*)
 
         /* Support C constructors, and C destructors in both user code
@@ -167,6 +168,7 @@ SECTIONS
         _srelocate = .;
         *(.ramfunc .ramfunc.*);
         *(.data .data.*);
+        KEEP (*(SORT(.xfa.*)))
         KEEP (*(.openocd .openocd.*))
         . = ALIGN(4);
         _erelocate = .;

cpu/cortexm_common/ldscripts/xfa.ld

@@ -0,0 +1 @@
+/* empty file overriding core/ldscripts/xda.ld */