cmd/compile: adjust stack slot alignment requirements on arm64

Currently any variable that is spilled onto the stack will occupy at least 8 bytes,
because the stack offset is required to be aligned with 8 bytes on linux/arm64.
This CL removes this constraint by aligning the stack slot with its actual size.

Updates #42385

Change-Id: Icbd63dc70cd19852802e43f134355f19ba7e1e29
Reviewed-on: https://go-review.googlesource.com/c/go/+/267999


Trust: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>