x86: drop SSE4a from SSE check again

x86: drop SSE4a from SSE check again

Upon re-consideration in commit 569d50f1c611 ("x86: further refine SSE
check (SSE4a, SHA, GFNI)") I went too far: Mixing of SSE and AVX insns
doesn't suffer as bad a penalty on AMD CPUs as on Intel ones. SSE4a
being an AMD-only extension, it shouldn't be part of the ISA extensions
set for which the diagnostic may get issued. Undo that part.