x86emul: suppress default test harness build with incapable assembler

x86emul: suppress default test harness build with incapable assembler

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, {evex}
insn pseudo-prefixes in, again in particular, test_x86_emulator.c causes
this build to fail though when the assembler is not new enough. Take
another big hammer and suppress the default harness build altogether
also when this and other pseudo-prefixes are not supported by the
specified (or defaulted to) assembler.

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