Exclude Perl_my_setlocale symbol where it's not defined.

Because makedef.pl doesn't take into account the various ifdefs
in embed.fnc, it must repeat their logic by maintaining various
sets of platform-specific inclusions and exclusions.

Perl_my_setlocale was added in b385bb4ddcb for Win32 only, so we
shouldn't be telling the linker to export it except on Win32.