[PPC] Remove Darwin support from POWER backend. This patch issues an error message if Darwin ABI is attempted with the PPC backend. It also cleans up existing test cases, either converting the test to use an alternative triple or removing the test if the coverage is no longer needed.
The majority of test cases were updated to use a different triple that does not
include the Darwin ABI. Many tests were also updated to use FileCheck, in place
llvm/test/tools/dsymutil/PowerPC/sibling.test was originally added to test
specific functionality of dsymutil using an object file created with an old
version of llvm-gcc for a Powerbook G4. After a discussion with @JDevlieghere he
suggested removing the test.
llvm/test/CodeGen/PowerPC/combine_loads_from_build_pair.ll was converted from a
PPC test to a SystemZ test, as the behavior is also reproducible there.
All other tests that were deleted were specific to the darwin/ppc ABI and no
Phabricator Review: https://reviews.llvm.org/D50988