runtime: change 386 startup convention

runtime: change 386 startup convention

Now the default startup is that the program begins at _rt0_386_$GOOS,
which behaves as if calling main(argc, argv). Main jumps to _rt0_386.

This makes the _rt0_386 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.

386 analogue of https://golang.org/cl/7525043

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7551045