gallivm: Use llvm.fabs.
gallivm: Use llvm.fabs.

Exactly the same code.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler