Fix rint for SSE/NEON.

It seems *sometimes* with aggressive optimizations the combination
`psub(padd(a, b), b)` trick to force rounding is compiled away. Here
we replace with inline assembly to prevent this (I tried `volatile`,
but that leads to additional loads from memory).

Also fixed an edge case for large inputs `a` where adding `b` bumps
the value up a power of two and ends up rounding away more than
just the fractional part.  If we are over `2^digits` then just return
the input.  This edge case was missed in the test since the test was
comparing approximate equality, which was still satisfied.  Adding
a strict equality option catches it.