[libc++] Fix `regex_search` to match `$` alone with `match_default` flag (#77256)

Using `regex_search` with the regex_constant `match_default` and a
simple regex pattern `$` is expected to match general strings such as
_"a", "ab", "abc"..._ at `[last, last)` positions. But, the current
implementation fails to do so.

Fixes #75042