cmd/compile: adjust stack slot alignment requirements on arm64
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 <68c7b5ebe4a86ef7e52157f5950041656bf82310@arm.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go