radeonsi: use the correct target machine when building shader variants

radeonsi: use the correct target machine when building shader variants

If the shader selector is created with a different context than
the shader variant, we should use the calling context's target machine
for the shader variant.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99419

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>