cmd/compile: reference ABIInternal memequal_varlen

memequal_varlen is put into a closure and is called in internal
ABI in the runtime. Emit an ABIInternal reference.

Updates #40724.

Change-Id: I914555f8188561882625e008b595389e50a3a167
Reviewed-on: https://go-review.googlesource.com/c/go/+/307233


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>