Fix up regcomp/regexec

Fix up regcomp/regexec

The problem is that parse_bracket_symbol is miscompiled, and it turns
out it is because of an incorrect attribute on re_string_fetch_byte_case.
Unlike re_string_peek_byte_case, this one is really not pure, it modifies memory
(increments pstr->cur_idx), and with the pure attribute GCC assumed it doesn't
and it cached the presumed value of regexp->cur_idx in a variable across the
 for (;; ++i)
   {
     if (i >= BRACKET_NAME_BUF_SIZE)
       return REG_EBRACK;
     if (token->type == OP_OPEN_CHAR_CLASS)
       ch = re_string_fetch_byte_case (regexp);
     else
       ch = re_string_fetch_byte (regexp);
     if (re_string_eoi(regexp))
       return REG_EBRACK;
     if (ch == delim && re_string_peek_byte (regexp, 0) == ']')
       break;
     elem->opr.name[i] = ch;
   }