[RESEND PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

[Vincent Mailhol <mailhol.vincent@wanadoo.fr>]

Aside of the __builtin_foo() ones, x86 does not directly rely on any
builtin functions.

However, such builtin functions are not explicitly deactivated,
creating some collisions, concrete example being ffs() from bitops.h,
c.f.:

| ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]
|   283 | static __always_inline int ffs(int x)

This patch adds -fno-builtin to KBUILD_CFLAGS for the x86
architectures in order to prevent shadowing of builtin functions.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
FYI, I tested this patch on a "make allyesconfig" for both x86_32 and
x86_64.

This is a resend. Only difference is that I dropped the RFC flag and
added Arnd in CC because he did a similar patch to fix ffs shadow
warnings in the past:

https://lore.kernel.org/all/20201026160006.3704027-1-arnd@kernel.org/
---
 arch/x86/Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index e84cdd409b64..5ff7b6571dd2 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -53,6 +53,8 @@ export REALMODE_CFLAGS
 # e.g.: obj-y += foo_$(BITS).o
 export BITS
 
+KBUILD_CFLAGS += -fno-builtin
+
 #
 # Prevent GCC from generating any FP code by mistake.
 #
-- 
2.34.1