gallivm: Reenable PPC VSX (v3)

gallivm: Reenable PPC VSX (v3)

Reenable the PPC64LE Vector-Scalar Extension for LLVM versions >= 3.8.1,
now that LLVM bug 26775 and its corollary, 25503, are fixed.

Amendment: remove extraneous spaces in macro def & invocations.

We would prefer a runtime check, e.g. via an LLVMQueryString
(analogous to glGetString, eglQueryString) or LLVMGetVersion API,
but no such API exists at this time.

Signed-off-by: Ben Crocker <bcrocker@redhat.com>
[Emil Velikov: remove LLVM_VERSION macro]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>