arm: move ARMv7 NEON stuff into isa/arm/neon

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/.

Note: other MRs add isa/riscv/ so arm/ will not be alone there.