tests/tcg: isolate from QEMU's config-host.mak

Do not include variables for the QEMU's own compiler, as they
are not necessarily related to the cross compiler used for tests/tcg.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-16-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-19-alex.bennee@linaro.org>