gallium: block signals for new thread when spawning threads

gallium: block signals for new thread when spawning threads

I'm hard pressed to think of any reason a gallium thread would want to
receive a signal, especially considering its probably loaded as a library
and you don't want the threads interfering with the main threads signal
handling.

This solves a problem loading llvmpipe into the X server for AIGLX,
where the X server relies on the SIGIO signal going to the main thread,
but once llvmpipe loads the SIGIO can end up in any of its threads.

Signed-off-by: Dave Airlie <airlied@redhat.com>