[docs] Add links to XMOS XCore documentation.

Summary: Add links to XCore ISA and ABI documents.

CC: llvm-commits, rafael

Differential Revision: http://llvm-reviews.chandlerc.com/D2981

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203936 91177308-0d34-0410-b5e6-96231b3b80d8