runtime/pprof, syscall: report MaxRSS on all unix platforms

All unix platforms currently supported by Go provide the getrusage
syscall. On aix and solaris the Getrusage syscall wrapper is not
available yet, so add and use it to report MaxRSS in memory profiles.

Change-Id: Ie880a3058171031fd2e12ccf9adfb85ce18858b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/391434


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>