[XRay] [compiler-rt] reformat and clarify CMakeLists.txt [NFC]

Summary:
- last change (+ the Apple support change) missed a lot of indentation
- shorten architecture SOURCES definitions as most fit 1 line/arch
- comment in English what's where, and where the different .a come from
  (using only the word "runtime" in the comment isn't useful, since the
  CMake primitive itself says "runtime" in its name)
- skip unsupported architectures quickly, to avoid extra indentation

Reviewers: dberris, eizan, kpw

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45568

llvm-svn: 329998