linux-user, alpha: fix oldumount syscall

linux-user, alpha: fix oldumount syscall

When we try to bootstrap debian/lenny for alpha, it fails because
it cannot umount /.root directory:

  ...
  Setting up initscripts (2.86.ds1-61) ...
  umount: /.root: Function not implemented
  dpkg: error processing initscripts (--configure):
   subprocess post-installation script returned error exit status 1
  dpkg: sysvinit: dependency problems, but configuring anyway as you request:
   sysvinit depends on initscripts; however:
    Package initscripts is not configured yet.

This is because, when we switched from syscall_nr.h to syscall.tbl,
the syscall #321 has been renamed from umount to oldumount and
syscall.c has not been updated to manage the new name.

oldumount has been introduced in linux 2.1.116pre1 by:
  7d32756b2 ("Import 2.1.116pre1")
...
 * We now support a flag for forced unmount like the other 'big iron'
 * unixes. Our API is identical to OSF/1 to avoid making a mess of AMD
...

Fixes: 6116aea994 ("linux-user, alpha: add syscall table generation support")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200502194642.32823-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>