gnu directives: add support for .gnu_attribute and .gnu_subsection in OAv2 context

This patch adds support for the GNU directives .gnu_attribute and
.gnu_subsection, used respectively for OAv1 and OAv2, and for OAv2
only. These directives behave like their AEABI counterparts, as
they are aliases intended for use by any backends supporting OAv1
and/or OAv2. Their availability is controlled by the TC_OBJ_ATTR_v1
and TC_OBJ_ATTR_v2 macros.

Previously, the "gnu_" subsection namespace was used only for
"gnu_testing_" and defaulted to a private scope. This patch updates
the scope recognition to correctly distinguish between private usage
(e.g., testing) and public usage (e.g., actual GNU subsections
storing public information).