gallivm: Override getHostCPUName() "generic" w/ "pwr8" (v4)

gallivm: Override getHostCPUName() "generic" w/ "pwr8" (v4)

If llvm::sys::getHostCPUName() returns "generic", override
it with "pwr8" (on PPC64LE).

This is a work-around for a bug in LLVM: a table entry for "POWER8NVL"
is missing, resulting in (big-endian) "generic" being returned on
little-endian Power8NVL systems.  The result is that code that
attempts to load the least significant 32 bits of a 64-bit quantity in
memory loads the wrong half.

This omission should be fixed in the next version of LLVM (4.0),
but this work-around should be left in place in case some
future version of POWER<n> also ends up unrepresented in LLVM's table.

This workaround fixes failures in the Piglit arb_gpu_shader_fp64 conversion
tests on POWER8NVL processors.

(V4: add similar comment in the code.)

Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Cc: 12.0 13.0 17.0 <mesa-stable@lists.freedesktop.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>