gallivm: Use LLVM global context.

gallivm: Use LLVM global context.

I saw that LLVM internally uses its global context for some things, even
when we use our own.  Given ours is also global, might as well use
LLVM's.

However, sepearate contexts can still be enabled with a simple source
code modification, for when the need/benefit arises.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>