regexec.c: utf8 doesn't match non-utf8 self

regexec.c: utf8 doesn't match non-utf8 self

Some regex patterns don't match a character with itself when the target
string is in utf8 and the pattern isn't, and the character is variant
under utf8.  (This means only Latin1-range characters in the pattern are
affected.)

The solution is to test for this case and use the utf8 representation of
the pattern character for the comparison.