Add support for half-precision floating point operations.
Introduce `__extendhfsf2` and `__truncsfhf2` in std/special/compiler_rt.
Add `__gnu_h2f_ieee` and `__gnu_f2h_ieee` as aliases that are used in
Windows builds.
The logic in std/special/compiler_rt/extendXfYf2.zig has been reworked
and can now operate on 16 bits floating point types.
`extendXfYf2()` and `truncXfYf2()` are marked `inline` to work around
a not entirely understood stack alignment issue on Windows when calling
the f16 versions of the builtins.
closes#1122
Ben Noordhuis
Andrew Kelley