MFC r328305: libcxxrt: Move mangled symbols out of extern "C++" in Version.map

MFC r328305: libcxxrt: Move mangled symbols out of extern "C++" in Version.map

r260553 added a number of mangled C++ symbols to Version.map inside of
an existing `extern "C++"` block.

ld.bfd 2.17.50 treats `extern "C++"` permissively and will match both
mangled and demangled symbols against the strings in the version map
block.  ld.lld interprets `extern "C++"` strictly, and matches only
demangled symbols.

I believe lld's behaviour is correct.  Contemporary versions of ld.bfd
also behave as lld does, so move the mangled symbols out of the
`extern "C++"` block.

PR:		225128, 185663
Sponsored by:	The FreeBSD Foundation