gallivm: don't use pabs intrinsic with llvm version >= 6

gallivm: don't use pabs intrinsic with llvm version >= 6

The intrinsic is gone, causing shader compilation to crash.
While here, also change the fallback code to match what llvm's auto-updater
of these intrinsics would do (except that there will still be zext/trunc
instructions in there), which should ensure that the sequence gets recognized
and fused back into a pabs in the end (I didn't test this, and it's possible
even the old sequence would get recognized, but I don't see a reason why we
shouldn't use the same sequence in any case).

Tested-by: Vinson Lee <vlee@freedesktop.org>