tcg/plugins: enable by default for most TCG builds

tcg/plugins: enable by default for most TCG builds

Aside from a minor bloat to file size the ability to have TCG plugins
has no real impact on performance unless a plugin is actively loaded.
Even then the libempty.so plugin shows only a minor degradation in
performance caused by the extra book keeping the TCG has to do to keep
track of instructions. As it's a useful feature lets just enable it by
default and reduce our testing matrix a little.

We need to move our linker testing earlier so we can be sure we can
enable the loader module required. As we have ruled out static &
plugins in an earlier patch we can also reduce the indent a little.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210709143005.1554-33-alex.bennee@linaro.org>