ParseARM64Suffix is not used outside cmd/asm/internal/arch.

Change-Id: I8e7782dce11cf8cd2fd08dd17e555ced8d87ba24
Reviewed-on: https://go-review.googlesource.com/105115
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>