DynaLoader: dl_dyld.xs: don't undef bool

I expect this was added when we added our own bool, and before
we used bool so liberally ourselves.  Now dl_dyld.xs fails to
compile, since MY_CXT_CLONE indirectly uses UNLIKELY() which
casts to bool.

Tested locally with:

  ./Configure -des -Dusedevel -Dusethreads -Ddlsrc=dl_dyld.xs

on a modern Darwin, which failed before this change and builds
after.

Based on work done by Sevan Janiyan in #21751.

Fixes #21751

ext/DynaLoader/dl_dyld.xs

@@ -48,7 +48,6 @@ been tested on NeXT platforms.
 #include "dlutils.c"	/* for SaveError() etc */
 
 #undef environ
-#undef bool
 #import <mach-o/dyld.h>
 
 static char *dlerror()