x86/build: Disable SSE4a

Leyvi Rose reported that his X86_NATIVE_CPU=y build is failing because our
instruction decoder doesn't support SSE4a and the AMDGPU code seems to be
generating those with his compiler of choice (CLANG+LTO).

Now, our normal build flags disable SSE MMX SSE2 3DNOW AVX, but then
CC_FLAGS_FPU re-enable SSE SSE2.

Since nothing mentions SSE3 or SSE4, I'm assuming that -msse (or its negative)
control all SSE variants -- but why then explicitly enumerate SSE2 ?

Anyway, until the instruction decoder gets fixed, explicitly disallow SSE4a
(an AMD specific SSE4 extension).

Fixes: ea1dcca ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Arisu Tachibana <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Harry Wentland <[email protected]>
Cc: <[email protected]>

Author: Peter Zijlstra

arch/x86/Makefile

@@ -75,7 +75,7 @@ export BITS
 #
 #    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
 #
-KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
+KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a
 KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
 KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2