Allow automatic long name macro generation

Prior to this commit, a macro that doesn't deal with thread context can
have its long 'Perl_foo' name form be automatically generated.  This was
added in 5.41 by GH #22691.  I had thought it worked in all cases, but
the threaded case had to be reverted by #23209.

The original scheme was too simplistic.  This new commit reintroduces
the thread context automatic handling, but isn't so naive this time.

This commit will enable the emptying of much of mathoms.c, and the
removal of quite a few similar functions scattered throughout the code.

The next commit will convert the first such function, as a proof of
concept.

Author: khwilliamson

embed.fnc

@@ -447,8 +447,7 @@
 :        created that #defines 'foo' as 'Perl_foo'.  This can be used to make
 :        any macro have a long name, perhaps to avoid name collisions.  If
 :        instead you define the macro as 'PERL_FOO' (all uppercase), the
-:        embed.h entry will use all uppercase.  Without the T flag the behavior
-:        is subject to change when both 'm' and 'p are specified.
+:        embed.h entry will use all uppercase.
 :
 :         suppress proto.h entry (actually, not suppressed, but commented out)
 :         suppress entry in the list of exported symbols available on all

regen/embed.pl

@@ -514,7 +514,40 @@ sub embed_h {
         if ($flags =~ /m/ && $flags =~ /p/) {
             my $full_name = full_name($func, $flags);
             next if $full_name eq $func;    # Don't output a no-op.
+
+            # Yields
+            #   #define Perl_func  func
+            # which works when there is no thread context.
             $ret = indent_define($full_name, $func, $ind);
+
+            if ($flags !~ /[T]/) {
+
+                # But when there is the possibility of a thread context
+                # parameter, $ret works only on non-threaded builds
+                my $no_thread_full_define = $ret;
+
+                # And we have to do more when there are threads.  First,
+                # convert the input argument list to 'a', 'b' ....  This keeps
+                # us from having to worry about all the extra stuff in the
+                # input list; stuff like the type declarations, things like
+                # NULLOK, and pointers '*'.
+                my $argname = 'a';
+                my @stripped_args;
+                push @stripped_args, $argname++ for $args->@*;
+                my $arglist = join ",", @stripped_args;
+
+                # In the threaded case, the Perl_ form is expecting an aTHX
+                # first argument.  Use mTHX to match that, which isn't passed
+                # on to the short form name, as that is expecting an implicit
+                # aTHX.  The non-threaded case just uses what we generated
+                # above for the /T/ flag case.
+                $ret = "#${ind}ifdef USE_THREADS\n"
+                     . "#${ind}  define $full_name(mTHX,$arglist)"
+                     .           "  $func($arglist)\n"
+                     . "#${ind}else\n"
+                     . "$ind  $no_thread_full_define" # No \n because no chomp
+                     . "#${ind}endif\n";
+            }
         }
         elsif ($flags !~ /[omM]/) {
             my $argc = scalar @$args;