MFC r367323:

Update libcxxrt's private copy of elftoolchain demangler

This updates the private copy of libelftc_dem_gnu3.c in libcxxrt with
the most recent version from upstream r3877. Similar to r367322, this
fixes a number of possible assertions, and allows it to correctly
demangle several names that it could not handle before.

PR: 250702

MFC r367337:

Make vector-related functions in libcxxrt's demangler static

Follow-up to r367323 by re-adding static to a number of the functions
copied from elftc's libelftc_vstr.c. This was requested by upstream.

PR: 250702