Add the "-trimpath" flag from Go 1.13.
Change-Id: I10938fb5f7efd701f4025e451ddf438dc3e011ee
GitHub-Last-Rev: 4e82381342e2c13c318514800bc82af7f6ed9e1a
GitHub-Pull-Request: golang/mobile#39
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/196877
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>