go/ssa/interp: add several intrinsics for Darwin

Also: drop all pretense of support for platforms other than darwin and linux.
This package is just a test of go/ssa, not a portable interpreter, and these
are the only platforms to which I have easy access.

Builds on: freebsd darwin plan9 linux windows

Change-Id: I965abc67b1280d33e933b83607a4372d65e070cf
Reviewed-on: https://go-review.googlesource.com/33163
Reviewed-by: Robert Griesemer <gri@golang.org>
9 files changed