Previously, gomobile init assumed that the XCode developer tools were available when running on darwin. That is not always the case, in particular for Android developers on macOS. Replace the GOOS check with an explicit check for the xcrun binary. Change-Id: Ie5ae917288932cc641a17f904ed9822a105367cc Reviewed-on: https://go-review.googlesource.com/35852 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>