[libc] add basic strtold implementation

Due to the differences between the types of long double, this function
is effectively three functions in one. This patch adds basic support for
the types of long double, although it's just using the fast path and the
fallback for the moment. I still need to implement a version of
Eisel-Lemire for performance, but the existing algorithms should be
correct.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D113710