radeon-llvm: Start multithreaded before using llvm.

radeon-llvm: Start multithreaded before using llvm.

This is required to make some of llvm's api calls
thread save. In particular the PassRegistry, which is
implicitly accessed while compiling shader programs.
The PassRegistry uses a mutex that is only active if
the llvm_is_multithreaded() returns true.
Calling llvm_start_multithreading() makes this happen
and by calling this function we try to make sure that
we can savely compile shaders in paralell.
Since there is also a call llvm_stop_multithreading()
in the llvm api, we cannot guarantee that this does
not get switched off while we are relying on this being
set, but for the easier use cases this fixes a race with
the radeon llvm compiler we have as of today.

Signed-off-by: Mathias Froehlich <Mathias.Froehlich@web.de>
Signed-off-by: Tom Stellard <thomas.stellard@amd.com>