mesa: Reduce libGL.so binary size by about 15%

mesa: Reduce libGL.so binary size by about 15%

This patch significantly reduces the size of the libGL.so binary. It does
not change the (externally visible) behavior of libGL.so at all.

gl_gentable.py generates a function, _glapi_create_table_from_handle.
This function allocates a large dispatch table, consisting of 1300 or so
function pointers, and fills this dispatch table by doing symbol lookups
on a given shared library.  Previously, gl_gentable.py would generate a
single, very large _glapi_create_table_from_handle function, with a short
cluster of lines for each entry point (function).  The idiom it generates
was a NULL check, a call to snprintf, a call to dlsym / GetProcAddress,
and then a store into the dispatch table.  Since this function processes
a large number of entry points, this code is duplicated many times over.

We can encode the same information much more compactly, by using a lookup
table.  The previous total size of _glapi_create_table_from_handle on x64
was 125848 bytes.  By using a lookup table, the size of
_glapi_create_table_from_handle (and the related lookup tables) is reduced
to 10840 bytes.  In other words, this enormous function is reduced by 91%.
The size of the entire libGL.so binary (measured when stripped) itself drops
by 15%.

So the purpose of this change is to reduce the binary size, which frees up
disk space, memory, etc.

size lib/libGL.so.1.2.0 on my system shows (Andreas)
   text	   data	    bss	    dec	    hex	filename
 565947	  11256	   2720	 579923	  8d953	lib/libGL.so.1.2.0 before
 469211	  21848	   2720	 493779	  788d3	lib/libGL.so.1.2.0 after

v2: Incorporate Matt's feedback.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Tested-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>