[sanitizer] Add early call handling to strchr + strrchr interceptors

Summary:
The strchr and strrchr interceptors are sometimes invoked too early
for their REAL() counterparts to be initialized.  We have seen this in
hooks invoked from tcmalloc on the dlsym() used in initializing
interceptors.  A special check is added to use internal_ routines for
this situation.

Reviewers: vitalybuka, aizatsky, filcab

Subscribers: filcab, llvm-commits, eugenis, kcc, zhaoqin, aizatsky, kubabrecka

Differential Revision: http://reviews.llvm.org/D19607

llvm-svn: 267793