Fix makeSlice
Switch to using the cabi abstraction to form function prologues and returns
Add missing void return
Build the __go_init_main function correctly

The semantics of the individual import functions in the gccgo ABI are different
to those originally modeled by the synthesized init function produced by
go/ssa.  We now ask go/ssa to produce gccgo-style import functions and build
the __go_init_main function using priorities extracted from the import data.

This depends on two go.tools patches which are under review:
https://codereview.appspot.com/78740043
https://codereview.appspot.com/78780043
Add a test script

This script lets me quickly see how many tests are newly passing/failing.