KVM: selftests: Don't force -march=x86-64-v2 if it's unsupported

KVM: selftests: Don't force -march=x86-64-v2 if it's unsupported

Force -march=x86-64-v2 to avoid SSE/AVX instructions if and only if the
uarch definition is supported by the compiler, e.g. gcc 7.5 only supports
x86-64.

Fixes: 9a400068a158 ("KVM: selftests: x86: Avoid using SSE/AVX instructions")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-and-tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241031045333.1209195-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>