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

In the next patch, I have 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.
Remove the non-inline function S_croak_memory_wrap from inline.h.

This appears to resolve these three related tickets:

    [perl #116989] S_croak_memory_wrap breaks gcc warning flags detection
    [perl #117319] Can't include perl.h without linking to libperl
    [perl #117331] Time::HiRes::clock_gettime not implemented on Linux (regression?)

This patch changes S_croak_memory_wrap from a static (but not inline)
function into an ordinary exported function Perl_croak_memory_wrap.
This has the advantage of allowing programs (particuarly probes, such
as in cflags.SH and Time::HiRes) to include perl.h without linking
against libperl.  Since it is not a static function defined within each
compilation unit, the optimizer can no longer remove it when it's not
needed or inline it as needed.  This likely negates some of the savings
that motivated the original commit 380f764c1ead36fe3602184804292711.
However, calling the simpler function Perl_croak_memory_wrap() still
does take less set-up than the previous version, so it may still be a
slight win.  Specific cross-platform measurements are welcome.