[xcore] Only support the 'm' inline assembly memory constraint. NFC.

Summary:
XCore doesn't seem to have any additional constraints. Therefore remove
the target hook.

No functional change intended.

Reviewers: friedgold

Reviewed By: friedgold

Subscribers: friedgold, llvm-commits

Differential Revision: http://reviews.llvm.org/D8921

llvm-svn: 237442