[XCore] Target optimized library function __memcpy_4()

Summary:
If the src, dst and size of a memcpy are known to be 4 byte aligned we
can call __memcpy_4() instead of memcpy().

Reviewers: robertlytton

Reviewed By: robertlytton

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2871

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202395 91177308-0d34-0410-b5e6-96231b3b80d8