move functions marked as mathomed in embed.fnc to mathoms.c

move functions marked as mathomed in embed.fnc to mathoms.c

Ever since commit 075eb5c9b6 mathom functions must be in mathoms.c for
their symbols to be skipped in makedef.pl on Win32 Perl. If a function is
marked 'b' in embed.fnc, regen.pl does NOT add its prototype to proto.h
(it is commented out). Without the proto.h entry, EXTERN_C will be missing
and a -DNO_MATHOMS + C++ Win32 Perl build will not link, since the C
function will have a mangled name and the symbol will not be found for
creating the perl linking library.

Also add EXTERN_C to Win32CORE, the init_Win32CORE symbol is special cased
for exporting in makedef.pl.

Perl_is_utf8_char_buf was marked as 'b' in commit 3cedd9d930
Perl_sv_copypv was marked as 'b' in commit 4bac9ae47b