[ExecutionEngine] Re-enable JIT tests on PowerPC.

These were previously re-enabled in d771f541, but had to be disabled again
in 2060a72b due to test failures.

This is a next step to landing https://reviews.llvm.org/D148192, which adds
a skeleton JITLink backend for PowerPC.

The fixes for those failures were (1) to explicitly specify IsLittleEndian =
true for the MachO YAML testcases, (2) disable some example tests for examples
that aren't supported on PowerPC yet, and (3) fixing the endianness of a
relocation read/write (for ELF R_AARCH64_TSTBR14) in RuntimeDyldELF.