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 <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go