[PPC64] Add support for the ICBT instruction on POWER8.

Patch by Kit Barton.

Support for the ICBT instruction is currently present, but limited to
embedded processors. This change adds a new FeatureICBT that can be used
to identify whether the ICBT instruction is available on a specific processor.

Two new tests are added:
 * Positive test to ensure the icbt instruction is present when using
-mcpu=pwr8
 * Negative test to ensure the icbt instruction is not generated when
using -mcpu=pwr7

Both test cases use the Prefetch opcode in LLVM. They are based on the
ppc64-prefetch.ll test case.

llvm-svn: 226033