x86emul: suppress default test harness build with incapable compiler

x86emul: suppress default test harness build with incapable compiler

A top level "make build", as used e.g. by osstest, wants to build all
"all" targets in enabled tools subdirectories, which by default also
includes the emulator test harness. The use of, in particular, AVX512
insns in, again in particular, test_x86_emulator.c causes this build to
fail though when the compiler is not new enough. Take a big hammer and
suppress the default harness build altogether when any of the extensions
used is not supported by the specified (or defaulted to) compiler.

Leave the "run" target alone though: While some of the test code blobs
may fail to build with older compilers, as long as the main executable
can be built some limited testing can still be done.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>