[compiler-rt] Add common interceptor for wcrtomb.

Summary: Currently there is a libc++ test failing under MSAN because wcrtomb is not intercepted. This patch adds an interceptor for it. 

Reviewers: samsonov, eugenis

Subscribers: tberghammer, danalbert, srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D12311

llvm-svn: 245994