syscall: remove Flock for aix/ppc64

CL 152397 removed it from gc's syscall package.

Updates golang/go#29084

Change-Id: Iac26d912851cdf34c208b9b7eb02b522c085c5ef
Reviewed-on: https://go-review.googlesource.com/c/152557
Reviewed-by: Ian Lance Taylor <iant@golang.org>