runtime: change amd64 startup convention

runtime: change amd64 startup convention

Now the default startup is that the program begins at _rt0_amd64_$GOOS,
which sets DI = argc, SI = argv and jumps to _rt0_amd64.

This makes the _rt0_amd64 entry match the expected semantics for
the standard C "main" function, which we can now provide for use when
linking against a standard C library.

R=golang-dev, devon.odell, minux.ma
CC=golang-dev
https://golang.org/cl/7525043