cmd/compile: split exported/non-exported methods for interface type

Currently, mhdr/methods is emitted with the same len/cap. There's no way
to distinguish between exported and non-exported methods statically.

This CL splits mhdr/methods into two parts, use "len" for number of
exported methods, and "cap" for all methods. This fixes the bug in
issue #22075, which intends to return the number of exported methods but
currently return all methods.

Note that with this encoding, we still can access either
all/exported-only/non-exported-only methods:

	mhdr[:cap(mhdr)]          // all methods
	mhdr                      // exported methods
	mhdr[len(mhdr):cap(mhdr)] // non-exported methods

Thank to Matthew Dempsky (@mdempsky) for suggesting this encoding.

Fixes #22075

Change-Id: If662adb03ccff27407d55a5578a0ed05a15e7cdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/259237


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>