tests/plugins: don't unconditionally add -Wpsabi

tests/plugins: don't unconditionally add -Wpsabi

Not all compilers support the -Wpsabi (clang-9 in my case). To handle
this gracefully we pare back the shared build machinery so the
Makefile is relatively "standalone". We still take advantage of
config-host.mak as configure has done a bunch of probing for us but
that is it.

Fixes: bac8d222a
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200713200415.26214-6-alex.bennee@linaro.org>