regcharclass.pl: Add ability to avoid wrong mnemonic

A future commit will pass this function data that shouldn't be
translated into a mnemonic, like 'f' for the letter f.  The reason is
that that code will potentially be executed on a machine with a
different character set than what the mnemonic would be valid for.

regcharclass.h

@@ -3765,6 +3765,6 @@
  * 696e706fddd3ce8cd48c7ea91caf4c9edf5c296432d320aa7b78631f69aa9eac lib/unicore/mktables
  * 50b85a67451145545a65cea370dab8d3444fbfe07e9c34cef560c5b7da9d3eef lib/unicore/version
  * 0a6b5ab33bb1026531f816efe81aea1a8ffcd34a27cbea37dd6a70a63d73c844 regen/charset_translations.pl
- * 02415e05b0af39e3fb07a1ec2050ab35787e28b258d926074b95f196dd922946 regen/regcharclass.pl
+ * b4f1b6922eae1fbac74236d06906225702f8f42b315809b1d9e2c613ab4ceb24 regen/regcharclass.pl
  * b2f896452d2b30da3e04800f478c60c1fd0b03d6b668689b020f1e3cf1f1cdd9 regen/regcharclass_multi_char_folds.pl
  * ex: set ro: */

regen/regcharclass.pl

@@ -309,6 +309,7 @@ sub val_fmt
 {
     my $self = shift;
     my $arg = shift;
+    my $always_hex = shift // 0;    # Use \x{}; don't look for a mnemonic
 
     # Format 'arg' using the printable character if it has one, or a %x if
     # not, returning a string containing the result
@@ -317,7 +318,7 @@ sub val_fmt
     return $hex_fmt unless defined $arg && $arg !~ /\D/;
 
     # We convert only things inside Latin1
-    if ($arg < 256) {
+    if (! $always_hex && $arg < 256) {
 
         # Find the ASCII equivalent of this argument (as the current character
         # set might not be ASCII)