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