makedef.pl shouldn't prepend Perl_ to symbols already starting with Perl_.

I had Perl_croak_memory_wrap defined in embed.fnc with the 'nroX' flags,
since this is a private function used by public macros.  I used the long
form of the name Perl_croak_memory_wrap everywhere, and used the 'o'
flag so that embed.h wouldn't contain a useless #define croak_memory_wrap
Perl_croak_memory_wrap.  Unfortunately, makedef.pl (used by the Win32
build process) didn't know what to do with that entry and created an
entry Perl_Perl_croak_memory_wrap.   Changing makedef.pl to use the 'o'
flag to decide whether to add the Perl_ prefix resulted in over 50 other
symbols changing in the output of makedef.pl.  I don't know if the changes
are correct or if the 'o' flag is in error on those entries in embed.fnc,
but I don't have time to check them all out.

This patch just stops makedef.pl from adding a Perl_ prefix if there is
already one there.