specs: correct mobile Cgo conclusion — iOS is not feasible either (#127)

An implementation attempt for #125 disproved the spec's iOS conclusion.
iOS fails on two independent, unfixable-from-here blockers:

- purego refuses to build for ios under CGO_ENABLED=0 (is_ios.go tripwire),
  so the library cannot compile Cgo-free once it imports purego/objc.
- Go's iOS port forces external (C) linking; even an empty program fails
  'GOOS=ios CGO_ENABLED=0 go build'. No iOS artifact links without a C toolchain.

The original feasibility check ('go build std') was a false positive: it
compiles packages but never links and never pulls in purego. Corrected
conclusion: both iOS and Android keep Cgo; no source change.

Author: changkun