[libc] Add a str() method to FPBits which returns a string representation.

Unit tests for the str() method have also been added.

Previously, a separate test only helper function was being used by the
test matchers which has regressed over many cleanups. Moreover, being a
test only utility, it was not tested separately (and hence the
regression).

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D150906