gdb/solib: rename convenience variable to _linker_namespace

gdb/solib: rename convenience variable to _linker_namespace

Based on feedback from IRC and PR solib/32959, this commit renames the
recently introduced convenience variable $_current_linker_namespace to
the shorter name $_linker_namespace. This makes it more in line with
existing convenience variables such as $_thread and $_inferior, and
faster to type.

It should be ok to simply change the name because the variable was never
released to the public, so there should be no existing scripts depending
on the name.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32959
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>