cmd/compile: default to -p=main for main package

cmd/compile: default to -p=main for main package

With the previous CL the compiler emits an unlinkable object if
the -p flag is not specified. It is actually okay (and convenient)
to omit the -p flag for (just) the main package. This CL makes it
so.

Change-Id: I978d54d14c45b3bb9ed7471e40a2c47f269b56f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/394834
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>