[gn build] Add build files for llvm/lib/Target/PowerPC + tests
The PowerPC target itself is similar to the X86 target in https://reviews.llvm.org/rL348903
The llvm-exegesis unittests bits are similar to the corresponding AArch64 in https://reviews.llvm.org/rL350499
The whole patch is very similar to the WebAssembly target being added in https://reviews.llvm.org/rL350628
Also add a dep from tools/llvm-exegesis/lib to the AArch64 subdir, which I
failed to do in r350499.
The motivation for this target is solely that it has a unit test and I want to
enable the GN<->CMake unittest syncing check for llvm.
Differential Revision: https://reviews.llvm.org/D56416