Add basic support for IBM Z (s390x) CPU feature (known as
'facilities') detection.

Note that some of these features are mandatory when using the Go
compiler (for example, ldisp and eimm) but aren't mandatory when
using gccgo.

Cryptographic function detection is not yet implemented for
gccgo.

Change-Id: Ic6494d0df0bc1c1ad1713c9ff11ae23fba03d215
Reviewed-on: https://go-review.googlesource.com/c/163003Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>