cmd/link/internal/arm64: increase the function alignment to 16

On arm64, a function's address is 16 bytes aligned, and
the assembler aligns the size of function symbol to 16 bytes,
so to keep the consistent, this patch changes the function
alignment in the linker to 16 bytes.

Change-Id: I4d1e89a56200453b7b586fe3f4656bada7544214
Reviewed-on: https://go-review.googlesource.com/c/go/+/225397


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>