alpha-linux-user: Fix umount syscall numbers

It has been pointed out on LKML that the alpha umount syscall numbers
are named wrong, and a patch to rectify that has been posted for 3.11.

Glibc works around this by treating NR_umount as NR_umount2 if
NR_oldumount exists.  That's more complicated than we need in QEMU,
given that we control linux-user/*/syscall_nr.h.

This is the last instance of TARGET_NR_oldumount, so delete that from
the strace.list.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>