Add support for other architectures in libc

[antoyo]

In order for me to be able to run libcore tests in m68k, it seems like I'll need libc to support this architecture.

Here's one place where such support will need to be added..

It will also need to support the other architectures supported by gcc for it to work on those platforms.

[glaubitz]

Support for m68k was added to the libc crate a while ago by me.

Support for hppa has been done by @hdeller:

• Add initial support for hppa-unknown-linux-gnu libc#3539

but we can't merge it unless rustc knows the hppa target:

• Revert "Add initial support for hppa-unknown-linux-gnu" libc#3542