[release-branch.go1.10] misc/cgo: fix test on iOS

The test in CL 123715 doesn't work on iOS, it needs to use a different
version scheme to determine whether SecKeyAlgorithm and friends exist.
Restrict the old version test to OSX only.

The same problem occurs on iOS: the functions tested don't exist before
iOS 10.  But we don't have builders below iOS 10, so it isn't a big issue.
If we ever get older builders, or someone wants to run all.bash on an
old iOS, they'll need to figure out the right incantation.

Update #25036

Change-Id: Ia3ace86b00486dc172ed00c0c6d668a95565bff7
Reviewed-on: https://go-review.googlesource.com/123959


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/124217


Reviewed-by: Ian Lance Taylor <iant@golang.org>