Rewrite subtarget feature support

[dylanmckay]

Rewrite subtarget feature code so that it works correctly.

[dylanmckay]

A hierarchy needs to be identified. We cannot have a predicate for every register and every function, and manually add each one by enumerating the datasheet for every MCU.

Patterns need to be identified

• Do all MCUs with SRAM have push/pop/etc?
• Does the presence of MUL imply MULS and MULSU?
• Does MUL[s][u] imply FMUL[s][u]?`

Does AVR-GCC's license allow us to use their subtarget feature code as a resource/reference?

[dylanmckay]

Resources:

• Wikipedia, AVR Instruction Set
• binutils/include/opcode/avr.h

[dylanmckay]

I have documented the inheritance tree here.