KVM: selftests: Fix build on on non-x86 architectures

Commit 9a400068 ("KVM: selftests: x86: Avoid using SSE/AVX
instructions") unconditionally added -march=x86-64-v2 to the CFLAGS used
to build the KVM selftests which does not work on non-x86 architectures:

  cc1: error: unknown value ‘x86-64-v2’ for ‘-march’

Fix this by making the addition of this x86 specific command line flag
conditional on building for x86.

Fixes: 9a400068 ("KVM: selftests: x86: Avoid using SSE/AVX instructions")
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>