Export std::get_new_handler() from libcxxrt.so

[arichardson]

Summary

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.

Obtained from: CheriBSD (https://github.com/CTSRD-CHERI/cheribsd/commit/f33ca065031020e307e21bef9a2074e777111f1e)

[emaste]

lib/libcxxrt/Version.map

329 ↗

(On Diff #31762)

For consistency with stdlibc++ this should go in GLIBCXX_3.4.20, but I don't really know why this is done.

[arichardson]

Use GLIBCXX_3.4.20

[brooks]

LGTM