runtime: mark arenaIdx.l1 and arenaIdx.l2 methods as nosplit

Mark the "l1" and "l2" methods on "arenaIdx" with //go:nosplit, since
these methods are called from a nosplit context (for example, from
"spanOf").

Fixes #56044.
Updates #21314.

Change-Id: I48c7aa756b59a13162c89ef21066f83371ae50f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/441859
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/mheap.go

@@ -576,6 +576,12 @@ func arenaBase(i arenaIdx) uintptr {
 
 type arenaIdx uint
 
+// l1 returns the "l1" portion of an arenaIdx.
+//
+// Marked nosplit because it's called by spanOf and other nosplit
+// functions.
+//
+//go:nosplit
 func (i arenaIdx) l1() uint {
 	if arenaL1Bits == 0 {
 		// Let the compiler optimize this away if there's no
@@ -586,6 +592,12 @@ func (i arenaIdx) l1() uint {
 	}
 }
 
+// l2 returns the "l2" portion of an arenaIdx.
+//
+// Marked nosplit because it's called by spanOf and other nosplit funcs.
+// functions.
+//
+//go:nosplit
 func (i arenaIdx) l2() uint {
 	if arenaL1Bits == 0 {
 		return uint(i)