alpha: add standard statfs64/fstatfs64 syscalls

As Joseph Myers points out, alpha has never had a standard statfs64
interface and instead returns only 32-bit numbers here.

While there is an old osf_statfs64 system call that returns additional
data, this has some other quirks and does not get used in glibc.

I considered making the stat64 structure layout compatible with
with the one used by the kernel on most other 64 bit architecture that
implement it (ia64, parisc, powerpc, and sparc), but in the end
decided to stay with the one that was traditionally defined in
the alpha headers but not used, since this is also what glibc
exposes to user space.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>