Revert "cmd/gomobile: avoid -buildmode for go install std"
Revert "cmd/gomobile: avoid -buildmode for go install std"

This reverts commit b84795b4945bb0ca4d464ffff3cba37d2ff049c5.

See golang/go#13246

Change-Id: Ie8b89a9ac6e5b6906b3a4de21122d694cf7a2d0a
Reviewed-on: https://go-review.googlesource.com/16917
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

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