syscall: remove Ustat

glibc 2.28 removes ustat.h and the ustat function entirely, which
breaks syscall.Ustat.

Updates golang/go#25990

Change-Id: I430802c86389a02cb4a0cd6caff36fa542d98f2f
Reviewed-on: https://go-review.googlesource.com/120535
Reviewed-by: Than McIntosh <thanm@google.com>
6 files changed