glsl: Add locking to builtin_builder singleton

glsl: Add locking to builtin_builder singleton

Consider a multithreaded program with two contexts A and B, and the
following scenario:

1. Context A calls initialize(), which allocates mem_ctx and starts
   building built-ins.
2. Context B calls initialize(), which sees mem_ctx != NULL and assumes
   everything is already set up.  It returns.
3. Context B calls find(), which fails to find the built-in since it
   hasn't been created yet.
4. Context A finally finishes initializing the built-ins.

This will break at step 3.  Adding a lock ensures that subsequent
callers of initialize() will wait until initialization is actually
complete.

Similarly, if any thread calls release while another thread is still
initializing, or calling find(), the mem_ctx/shader would get free'd while
from under it, leading to corruption or use-after-free crashes.

Fixes sporadic failures in Piglit's glx-multithread-shader-compile.

Bugzilla: https://bugs.freedesktop.org/69200
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "10.1 10.0" <mesa-stable@lists.freedesktop.org>