cmd/compile: clarify adjustctxt inlining comment

cmd/compile: clarify adjustctxt inlining comment

The reason why adjustctxt wasn't being inlined was reported as:

	function too complex: cost 92 exceeds budget 80

However, after tweaking the code to be under the budget limit, we see
the real blocker:

	non-leaf function

There is little we can do about this one in particular at the moment.
Create a section with funcs that will need mid-stack inlining to be
inlineable, since this will likely come up again in other cases.

Change-Id: I3a8eb1546b289a060ac896506a007b0496946e84
Reviewed-on: https://go-review.googlesource.com/65650
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>