This re-write was needed to fix deficiencies in the existing ldexp,
which was failing to compute correct results for both f16 and f80.
It would be nice to add a fast multiplication-based fallback in the
future for targets that have a hardware FPU, but this implementation
should be much faster than the existing for targets without one.
This also addresses a nit from #10133 where IntT might be a confusing
name because it might imply signed integer (iX, not uX). We settled on
TBits for math/float.zig so I've applied that change here too.
When I originally wrote ldexp() I copied the name from parse_hex_float.
We assume we are compiled on a base-2 radix floating point system. This
is a reasonable assumption. musl libc as an example also assumes this.
We implement scalbn as an alias for ldexp, since ldexp is defined as 2
regardless of the float radix. This is opposite to musl which defines
scalbn in terms of ldexp.
Closes#9799.
