Implement a bunch of custom glibc wrappers & corresponding tests.

- Introduce several new custom glibc wrappers
- Implement some of the not yet implemented wrappers
- Refactor and extend the tests
- Add script to make sure all declare custom glibc wrappers are implemented & tested.

Patch by Lorenzo Martignoni!

Differential Revision: http://llvm-reviews.chandlerc.com/D2234

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195381 91177308-0d34-0410-b5e6-96231b3b80d8