Move unlikely executed macro to function

Move unlikely executed macro to function

The bulk of this macro is extremely rarely executed, so it makes sense
to optimize for space, as it is called from a fair number of places, and
move as much as possible to a single function.

For whatever it's worth, on my system with my typical compilation
options, including -O0, the savings was 19640 bytes in regexec.o, 4528
in utf8.o, at a cost of 1488 in locale.o.