mesa: Replace a priori knowledge of gcc attributes with configure tests.

mesa: Replace a priori knowledge of gcc attributes with configure tests.

Note that I had to add support for testing the packed attribute to
m4/ax_gcc_func_attribute.m4.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com> [C bits]
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>