[libc] Add implementations of ldexp[f|l].

The rounding behavior of NormalFloat to float format has been changed
to round to nearest. Also, a bug in NormalFloat to subnormal number
conversion has been fixed.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D91591

GitOrigin-RevId: bb8f2585c6eab263916757435d71df16d92de4a8
17 files changed