i965: Break out backend compiler to its own library

i965: Break out backend compiler to its own library

This introduces a new libtool helper library, libi965_compiler.la.  This
library is moderately self-contained, but still needs to link to all of
libmesa.la among other things.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>