[XCore] Add support for the "m" inline asm constraint.

Summary:
This provides support for CP and DP relative global accesses in inline
asm.

Reviewers: robertlytton

Reviewed By: robertlytton

Differential Revision: http://llvm-reviews.chandlerc.com/D2943

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203129 91177308-0d34-0410-b5e6-96231b3b80d8