[gn build] Add build files for llvm/lib/Target/AArch64 + tests
The AArch64 target itself is similar to the X86 target in https://reviews.llvm.org/rL348903
The llvm-exegesis AArch64 bits are similar to the X86 bits in http://reviews.llvm.org/rL350184
The llvm-exegesis unittests AArch64 bits are similar to the X86 bits in https://reviews.llvm.org/rL350413
llvm/unittests/Target/AArch64 doesn't have an equivalent since the X86 Target
only has lit tests, no unittests.
Differential Revision: https://reviews.llvm.org/D56364