[MSAN] Fix test SmallPreAllocatedStackThread for MIPS

Summary: Msan was intercepting version 2.1 of the pthread_create function which was making it to crash in libc because __pthread_create_2_1 modifies the stack attributes of the thread. Intercepting the correct version fixes the test SmallPreAllocatedStackThread.

Reviewers: eugenis, samsonov
Subscribers: llvm-commits, mohit.bhakkad, jaydeep
Differential: http://reviews.llvm.org/D17603
llvm-svn: 261980