[tsan] Use REAL(malloc) instead of __libc_malloc for Android.

Summary:
Android doesn't have __libc_malloc and related allocation
functions. As its dynamic linker doesn't use malloc, so
we can use REAL(malloc) to replace __libc_malloc safely.

Reviewers: kcc, eugenis, dvyukov

Subscribers: llvm-commits, tberghammer, danalbert, srhines

Differential Revision: http://reviews.llvm.org/D15297

llvm-svn: 255167