This CL removes the last of the direct system calls in the runtime package.
This is the last CL for 1.11.

Use libcCall instead of asmcgocall in a few places I accidentally used
the wrong one.

For 1.12, we need to think about whether/how the syscall package
should be moved over to libc.

Update #17490

Change-Id: I4f0bd9cd6023f662f2e29588266fdfae5233898f
Reviewed-on: https://go-review.googlesource.com/118736
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>