[dev.typeparams] internal/bytealg: call memeqbody directly in memequal_varlen on ARM64

Currently, memequal_varlen opens up a frame and call memequal,
which then tail-calls memeqbody. This CL changes memequal_varlen
tail-calls memeqbody directly.

This makes it simpler to switch to the register ABI in the next
CL.

Change-Id: Ia1367c0abb7f4755fe736c404411793fb9e5c04f
Reviewed-on: https://go-review.googlesource.com/c/go/+/324399


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>