[flang] fix build on *BSD after 4762c655 (#86204)

The HUGE definition collides with the HUGE macro from math.h. Unlike the
fix in 3149c934 (#84478) (largely reverted in f95710c7), add
another #undef HUGE since there is no practical way to make FreeBSD's
headers not define HUGE and still define XSI interfaces such as isascii
or strnlen.

Update comments above `#undef HUGE` instances to reflect the fact that
all major BSD versions (I checked DragonFly, FreeBSD, NetBSD, and
OpenBSD) leak the HUGE macro from math.h to various degrees.

Fixes #86038