enable n32 and n64, and move o32 into mips/mips32

[Roland McGrath]

> If LinuxThreads won't compile with the tls.h in sysdeps/generic/,
> perhaps that one needs to change?  It seems wrong to require a tls.h
> for architectures that do not support TLS.

tls.h in linuxthreads has come to actual mean all the thread-local storage
accessors for the machine not just ELF TLS per se, i.e. THREAD_GETMEM et al
are there in at least most of the ports.