x/build: add Corellium iOS and Android builders

[eliasnaur]

@steeve from Zenly kindly gave us access to a Corellium account. Corellium allows us to run iOS and Android on virtual cloud devices with fewer restrictions than actual devices.

This issue tracks adding Corellium builders.

[gopherbot]

Change https://golang.org/cl/174202 mentions this issue: runtime/cgo: ignore missing Info.plist files on iOS

[gopherbot]

Change https://golang.org/cl/174300 mentions this issue: runtime: implement pthread functions for darwin/arm64

[gopherbot]

Change https://golang.org/cl/174307 mentions this issue: cmd/go,cmd/internal/sys,cmd/link: skip Go build ids for externally linked tools

[gopherbot]

Change https://golang.org/cl/174305 mentions this issue: cmd/dist: set the default external linker on platforms without gcc

[gopherbot]

Change https://golang.org/cl/174306 mentions this issue: cmd/dist: detect GOHOSTARCH on iOS

[gopherbot]

Change https://golang.org/cl/174308 mentions this issue: misc/ios: add clang wrapper for self-hosted iOS builds

[gopherbot]

Change https://golang.org/cl/174310 mentions this issue: time: look for zoneinfo.zip in GOROOT