radeon/llvm: Initialize gallivm targets when initializing the AMDGPU target v2

radeon/llvm: Initialize gallivm targets when initializing the AMDGPU target v2

This fixes a race condition in the glx-multithreaded-shader-compile
test.

v2:
  - Replace gallivm_init_llvm_{begin,end}() with gallivm_init_llvm_targets().

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
CC: "10.6 11.0" <mesa-stable@lists.freedesktop.org>