arm64: reduces the struct size of instructions

[mathetake]

This makes arm64's instruction data structure smaller, and
makes it more aligned with amd64. As a result, the compilation
gets slightly faster and uses 10% less memory space for
compiling wazero itself.

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
               │  old.txt   │             new.txt              │
               │   sec/op   │   sec/op    vs base              │
Compilation-10   3.444 ± 2%   3.315 ± 1%  -3.75% (p=0.001 n=7)

               │   old.txt    │              new.txt               │
               │     B/op     │     B/op      vs base              │
Compilation-10   391.2Mi ± 0%   354.7Mi ± 0%  -9.35% (p=0.001 n=7)

               │   old.txt   │              new.txt              │
               │  allocs/op  │  allocs/op   vs base              │
Compilation-10   693.3k ± 0%   695.3k ± 0%  +0.28% (p=0.001 n=7)