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: 9a40006 ("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>

Author: sean-jc

tools/testing/selftests/kvm/Makefile

@@ -249,8 +249,10 @@ ifeq ($(ARCH),s390)
 	CFLAGS += -march=z10
 endif
 ifeq ($(ARCH),x86)
+ifeq ($(shell echo "void foo(void) { }" | $(CC) -march=x86-64-v2 -x c - -c -o /dev/null 2>/dev/null; echo "$$?"),0)
 	CFLAGS += -march=x86-64-v2
 endif
+endif
 ifeq ($(ARCH),arm64)
 tools_dir := $(top_srcdir)/tools
 arm64_tools_dir := $(tools_dir)/arch/arm64/tools/