[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350629 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed