selftests/nolibc: explicitly enable ARM mode

The default could also be -mthumb.

Explicitly use -marm to keep everything predictable.

Tested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250301-nolibc-armthumb-v1-1-d1f04abb5f6d@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/testing/selftests/nolibc/Makefile

@@ -160,6 +160,7 @@ Q=@
 endif
 
 CFLAGS_i386 = $(call cc-option,-m32)
+CFLAGS_arm = -marm
 CFLAGS_ppc = -m32 -mbig-endian -mno-vsx $(call cc-option,-mmultiple)
 CFLAGS_ppc64 = -m64 -mbig-endian -mno-vsx $(call cc-option,-mmultiple)
 CFLAGS_ppc64le = -m64 -mlittle-endian -mno-vsx $(call cc-option,-mabi=elfv2)