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: ea1dcca1 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Arisu Tachibana <arisu.tachibana@miraclelinux.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Cc: <stable@kernel.org>