[libc] [llvm] [libc] Add maintainers file (PR #133471)

[Michael Jones via llvm-commits]

michaelrj-google wrote:

> @PiJoules for stdfix?

If he wants to I'm willing to add him, but he can also propose his own CL after this one has landed.

https://github.com/llvm/llvm-project/pull/133471