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 <[email protected]>
Cc: 12.0 13.0 17.0 <[email protected]>
Acked-by: Emil Velikov <[email protected]>
(cherry picked from commit b934aae36410e09892e772b26f61fed5d037fc69)

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