regexec.c: Fix locale and \s

regexec.c: Fix locale and \s

The handling for locale \s and \S both assume that the character in
ASCII platforms at 0x20 is a space.  This is not necessarily so.

I'm guessing that the code was originally just copied and pasted from
the non-locale space handling code without thinking.  That code hard-coded
in the space character, probably to avoid an expensive swash fetch for a
common situation.