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

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>