importing the latest libcxxrt snapshot

[Antti Kantee <pooka%iki.fi@localhost>]

Hi,

I want to update libcxxrt to the latest upstream 
(e64e93fe5bba67a6d52cbe5a97f8770c054bfa65).  The current one in the 
NetBSD tree is missing __cxa_throw_bad_new_array_length(), which is 
generated by gcc 4.9.  This is probably a no-brainer proposal, but since 
libcxxrt is in external, thought it would be polite to propose first. 
I'm also happy with just cherry-picking the __cxa_foo commits, in case 
that's preferred for whatever reason.

  - antti