[compiler-rt] Fix interception of memcpy/memmove on win64

Summary:
This patch is fixing running interception unittests for memcpy/memmove on
windows 64.

Reviewers: rnk

Subscribers: llvm-commits, wang0109, kubabrecka, chrisha

Differential Revision: https://reviews.llvm.org/D22641

llvm-svn: 276324