CMakeLists.txt: Enable -fno-common globally.

This is needed to force the .common type to .bss type.
This will force the .common variables to go into bss section
in the compiled object file.
As of now the movement to the .bss section was happening only
during the final linking stage.

The -fno-common option specifies that the compiler should place
uninitialized global variables in the data section of the object
file, rather than generating them as common blocks.  This has the
effect that if the same variable is declared (without "extern")
in two different compilations, you will get a multiple-definition
error when you link them.

e.g:
// file a.c
// file-scope

int b;

// file b.c
// file-scope

int b;

If there exist two non-extern declarations of the same variable
then no-common will cause a linker error.
This sequence would have compiled before, but will now cause a
linker error.

Signed-off-by: Adithya Baglody <[email protected]>

CMakeLists.txt

@@ -136,6 +136,7 @@ zephyr_compile_options(
   -imacros ${AUTOCONF_H}
   -ffreestanding
   -Wno-main
+  -fno-common
   ${NOSTDINC_F}
   ${TOOLCHAIN_C_FLAGS}
 )