Down the line, we probably should not have one directory per ISA
extension. This replicates the model from the hw/*/ directory.

isa/ seems better than arch/ because:
 - "Instruction Set Architecture" is more specific than "Architecture",
 - it saves one character,
 - no modules/ subdirectory starts with I yet (so this will not break
   auto-completion habits).

This also moves non-NEON-specific files to isa/arm/.