[libc] Move implementations of cosf, sinf, sincosf to src/math directory.

NFC intended in the implementaton. Only mechanical changes to fit the LLVM
libc implementation standard have been done.

Math testing infrastructure has been added. This infrastructure compares the
results produced by the libc with the high precision results from MPFR.
Tests making use of this infrastructure have been added for cosf, sinf and
sincosf.

Reviewers: abrachet, phosek

Differential Revision: https://reviews.llvm.org/D76825