lib/xref: work around GCC bug 41091

lib/xref: work around GCC bug 41091

gcc fucks up global variables with section attributes when they're used
in templated C++ code.  The template instantiation "magic" kinda breaks
down (it's implemented through COMDAT in the linker, which clashes with
the section attribute.)

The workaround provides full runtime functionality, but the xref
extraction tool (xrelfo.py) won't work on C++ code compiled by GCC.

FWIW, clang gets this right.

Signed-off-by: David Lamparter <equinox@diac24.net>