x86: HAVE_X86_LAHF_SAHF, HAVE_X86_MOVBE and -march=x86-64-vN (bug 28782)

HAVE_X86_LAHF_SAHF is implied by x86-64-v2, and HAVE_X86_MOVBE by
x86-64-v3.

The individual flag does not appear in -fverbose-asm flag output
even if the ISA level implies it.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>