cmd/compile: on 32-bit, bump up align for values that may contain 64-bit fields

cmd/compile: on 32-bit, bump up align for values that may contain 64-bit fields

On 32-bit systems, these need to be aligned to 8 bytes, even though the
typechecker doesn't tell us that.

The 64-bit allocations might be the target of atomic operations
that require 64-bit alignment.

Fixes 386 longtest builder.

Fixes #73173

Change-Id: I68f6a4f40c7051d29c57ecd560c8d920876a56a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/663015
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>