cmd/gomobile: don't ignore -openal if no NDK was found
cmd/gomobile: don't ignore -openal if no NDK was found

Updates golang/go#20902

Change-Id: I523c0b7a205e620c143f2ba9c7cdbe30865a9c59
Reviewed-on: https://go-review.googlesource.com/101136
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile