To use the NDK before version r19b standalone toolchains had to be
generated. Version r19b added prebuilt standalone toolchains.
Use the prebuilt for gomobile build and gomobile bind and
stop generating toolchains during gomobile init.
gomobile init is now only necessary for building OpenAL for
gomobile build programs.
This change is not compatible with NDK versions < r19b, but the
user is instructed how to upgrade when running gomobile build or
gomobile bind.
Change-Id: I96953298ecce42402459a9dd15169c09fe6b6f8b
Reviewed-on: https://go-review.googlesource.com/c/163378
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
ca80213619