[compiler-rt] Add internal wcslen to avoid crashing on windows 64-bits

Summary:
The function wcslen is incorrectly hooked on windows 64-bits.

The interception library is not able to hook without breaking the code.
The function is too small and the interception must be done with
trampoline-hooking which turned out to be incorrect on a small
loop (first few instructions have a backedge).

Reviewers: rnk

Subscribers: wang0109, chrisha, llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D22363

llvm-svn: 275488