build: are there situations where 'ar' does not work?

[Ralf Wildenhues]

Hello GCC developers,

Libtool recently added a configure test for @file support in ar
(and other tools) in order to avoid the slower partial building
or static libraries on some systems (and have an escape route on
systems where the archiver doesn't allow adding to an archive).

Now, to avoid issues when GCC uses newer Libtool: are there situations
where running $AR does not work?  Cross compile?  In-tree binutils?

Thanks,
Ralf