editors/libreoffice: fix build with lld 17

Building editors/libreoffice with lld 17 results in the following link
errors:

ld: error: version script assignment of 'PRIVATE_1.4' to symbol '_ZN3sal13backtrace_getEm' failed: symbol not defined
ld: error: version script assignment of 'GLIBCXX_3.4' to symbol '_ZNSs4_Rep20_S_empty_rep_storageE' failed: symbol not defined

Upstream fixed this only for Linux with:
https://github.com/LibreOffice/core/commit/baddc742c3fe260d20d7924ab134f0fbf179fd85

Add the same block to the more generic unxgcc.mk makefile, so it gets
picked up for FreeBSD too.

PR: 274697
Author: Dimitry Andric <dim@FreeBSD.org>