contrib/plugins: use an independent makefile

The initial reason to write this patch was to remove the last use of
CONFIG_DEBUG_TCG from the makefiles; the flags to use to build TCG
plugins are unrelated to --enable-debug-tcg, and instead they should
be the same as those used to build emulators (the plugins are not build
via meson for demonstration reasons only).

However, since contrib/plugins/Makefile is also the last case of doing
a compilation job using config-host.mak, go a step further and make it
use a completely separate configuration file, removing all references
to compilers from the toplevel config-host.mak.  Clean up references to
empty variables, and use .SECONDARY so that intermediate object files
are not deleted.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>