Darwin: add __sinpi (etc) and __exp10 libbuiltins

These functions have the same constness properties of the normal libm
functions, which allows LLVM to optimise code better in general. There
are also a couple of specific optimisations that only trigger when
these are properly marked.

rdar://problem/13729466

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199249 91177308-0d34-0410-b5e6-96231b3b80d8