cmd/compile/internal/gc: parse command-line flags earlier

cmd/compile/internal/gc: parse command-line flags earlier

This CL moves the call to base.ParseFlags() earlier in compiler
startup. This is necessary so CL 393715 can use base.Ctxt.Pkgpath to
construct types.LocalPkg.

Updates #51734.

Change-Id: I9f5f75dc9d5fd1b1d22e98523efc95e6cec64385
Reviewed-on: https://go-review.googlesource.com/c/go/+/406055
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>