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>

src/cmd/compile/internal/gc/inl_test.go

@@ -29,6 +29,10 @@ func TestIntendedInlining(t *testing.T) {
 	// be inlined.
 	want := map[string][]string{
 		"runtime": {
+			// TODO(mvdan): enable these once mid-stack
+			// inlining is available
+			// "adjustctxt",
+
 			"add",
 			"addb",
 			"adjustpanics",
@@ -58,8 +62,6 @@ func TestIntendedInlining(t *testing.T) {
 			"(*bmap).keys",
 			"(*bmap).overflow",
 			"(*waitq).enqueue",
-
-			//"adjustctxt", TODO(mvdan): fix and re-enable
 		},
 		"runtime/internal/sys": {},
 		"unicode/utf8": {