os,llvmpipe: Set rasterizer thread names on Linux.

os,llvmpipe: Set rasterizer thread names on Linux.

To help identify llvmpipe rasterizer threads -- especially when there
can be so many.

We can eventually generalize this to other OSes, but for that we must
restrict the function to be called from the current thread.  See also
http://stackoverflow.com/a/7989973

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