[libc] tweak strtof errno behavior

When strtof/d/ld return a subnormal number they may set errno to
ERANGE. This change makes this behavior more consistent by making any
decimal number converting to a subnormal set errno to ERANGE. This
brings it in line with hexadecimals, which currently only set errno to
ERANGE if the number is truncated when converting to a subnormal.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D113626

GitOrigin-RevId: 4cdf9884b60de758243c85a6ac1f94aa975fd10d
3 files changed