[compiler-rt] Fix minimal stack reserved for unittest on MSVC build

Summary:
Some unittest were not able to run in 64-bit because they need more than
2MB of stack and the default allocated stack with MSVC linker is 1MB.

Reviewers: rnk

Subscribers: wang0109, chrisha, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D22101

llvm-svn: 274785