Export std::get_new_handler() from libcxxrt.so

Export std::get_new_handler() from libcxxrt.so

When trying to build world for MIPS64 with clang I was getting
linker errors because of a missing reference to std::get_new_handler().
It turns out std::get_new_handler() was not listed in Version.map so it was
marked as a local symbol in libcxxrt.so.

Reviewed by:	theraven, brooks (mentor), emaste
Approved by:	trasz
Obtained from:	CheriBSD
Differential Revision:	https://reviews.freebsd.org/D11925