x86/boot/compressed: prefer cc-option for CFLAGS additions

as-option tests new options using KBUILD_CFLAGS, which causes problems
when using as-option to update KBUILD_AFLAGS because many compiler
options are not valid assembler options.

This will be fixed in a follow up patch. Before doing so, move the
assembler test for -Wa,-mrelax-relocations=no from using as-option to
cc-option.

Link: https://lore.kernel.org/llvm/CAK7LNATcHt7GcXZ=jMszyH=[e-mail]/
Suggested-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>
Tested-by: Linux Kernel Functional Testing <[e-mail]>
Tested-by: Anders Roxell <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

arch/x86/boot/compressed/Makefile

···
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 KBUILD_CFLAGS += -D__DISABLE_EXPORTS
 # Disable relocation relaxation in case the link is not PIE.
-KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
+KBUILD_CFLAGS += $(call cc-option,-Wa$(comma)-mrelax-relocations=no)
 KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
 
 # sev.c indirectly inludes inat-table.h which is generated during