Fix mac build w/echo -n

Soong doesn't wrap everything in bash, but relies on the ninja default
of /bin/sh. We should probably improve that in the future, but for now,
just fix the build.

On Mac, /bin/sh and /bin/bash are both bash, but /bin/sh implies `set -o
posix`, and ignores the `-n` flag.

Test: treehugger
Test: build on a mac
Change-Id: Icf41b1c5a1ce9eb0a56e39e68c433fc80b53620f