gdb/solib: Change type of convenience variable _current_linker_namespace

gdb/solib: Change type of convenience variable _current_linker_namespace

Based on IRC feedback since commit 6a0da68c036a85a46415aa0dada2421eee7c2269

    gdb: add convenience variables around linker namespace debugging

This commit changes the type of the _current_linker_namespace variable
to be a simple integer. This makes it easier to use for expressions,
like breakpoint conditions or printing from a specific namespace once
that is supported, at the cost of making namespace IDs slightly less
consistent.

This is based on PR solib/32960, where no negative feedback was given
for the suggestion.

The commit also changes the usage of "linkage namespaces" to "linker
namespaces" in the NEWS file, to reduce chance of confusion from an end
user.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32960
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>