mesa: Shrink the size of the enum string lookup struct.

mesa: Shrink the size of the enum string lookup struct.

Since it's only used for debug information, we can misalign the struct and
save the disk space.  Another 19k on a 64-bit build.

v2: Make a compiler.h macro to only use the attribute if we know we can.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@.intel.com>