syscall, cmd/go/internal/lockedfile: remove Flock syscall for aix/ppc64

AIX doesn't provide flock() syscall, it was previously emulated by fcntl
calls. However, there are some differences between a flock() syscall and
a flock() using fcntl. Therefore, it's safer to remove it and just
provide FcntlFlock.

Thus, lockedfile implementation must be moved to use FcntlFlock on aix/ppc64.

Updates #29065.
Fixes #29084.

Change-Id: Ic48fd9f315f24c2acdf09b91d917da131a1f2dd5
Reviewed-on: https://go-review.googlesource.com/c/152397


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>