Automatic merge from submit-queue

Reimplement 'pause' in C - smaller footprint all around

Statically links against musl. Size of amd64 binary is 3560 bytes.

I couldn't test the arm binary since I have no hardware to test it on, though I assume we want it to work on a raspberry pi.

This PR also adds the gcc5/musl cross compiling image used to build the binaries.

@thockin