[esan] Intercept calloc to avoid deadlocks with tcmalloc

Summary:
When tcmalloc initializes before esan, esan's initialization ends up
calling back into tcmalloc due to the calloc done by dlsym.  This results
in a deadlock.  We avoid this by special-casing this single allocation.

Intercepting calloc also gives us the opportunity to act on its zeroing as
stores by the application.

Reviewers: aizatsky

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D21086

llvm-svn: 272076