[gn build] Add build files for llvm/lib/Target/ARM + tests
The ARM target itself is similar to the X86 target in https://reviews.llvm.org/rL348903
The llvm-exegesis unittests ARM bits are similar to the X86 bits in https://reviews.llvm.org/rL350413
Both are similar to the corresponding AArch64 bits in https://reviews.llvm.org/rL350499 too
After this, everything in my local GN branch is upstreamed to LLVM.
Differential Revision: https://reviews.llvm.org/D56371