alpha: Use the generic gethostname

alpha: Use the generic gethostname

alpha was the only target implementing gethostname with the syscall
rather than through uname. Its only behavioural difference was the errno
for a too-small buffer: it reported EOVERFLOW where the generic
implementation, gethostname(2) and misc/tst-gethostname expect
ENAMETOOLONG, so alpha failed that test:

  tst-gethostname.c:96: numeric comparison failure
     left: 112 (0x70, EOVERFLOW); from: errno
    right: 63 (0x3f); from: ENAMETOOLONG

The file contains nothing but that function, so removing it lets the
sysdeps search fall through to sysdeps/posix/gethostname.c, which
produces the same buffer contents and the expected errno.
misc/tst-gethostname passes on alpha with it.

Suggested-by: Florian Weimer <fw@deneb.enyo.de>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>