tests/tcg: list test targets in Makefile.prereqs

Omit the rules altogether for targets that do not have a compiler.
Makefile.qemu now is only invoked if the tests are actually built/run.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-14-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-17-alex.bennee@linaro.org>
3 files changed