mesa: fix 32bit cross compilation on a 64bit machine

mesa: fix 32bit cross compilation on a 64bit machine

When cross compiling on a 64bit machine, gen_matypes.c is build
for the host machine (64bit) but must generates code for the target
machine (32bit). This causes wrong offsets all over the place and
crashes googleearth on my machine. Solution is to add -m32 when
cross compiling.

Attached patch is compatible with linux-x86-32 and autoconf based
builds.