More Arm64 lowering/backend work (https://github.com/Shopify/ruby/pull/307)

* More Arm64 lowering/backend work

* We now have encoding support for the LDR instruction for loading a PC-relative memory location
* You can now call add/adds/sub/subs with signed immediates, which switches appropriately based on sign
* We can now load immediates into registers appropriately, attempting to keep the minimal number of instructions:
  * If it fits into 16 bytes, we use just a single movz.
  * Else if it can be encoded into a bitmask immediate, we use a single mov.
  * Otherwise we use a movz, a movk, and then optionally another one or two movks.
* Fixed a bunch of code to do with the Op::Load opcode.
* We now handle GC-offsets properly for Op::Load by skipping around them with a jump instruction. (This will be made better by constant pools in the future.)
* Op::Lea is doing what it's supposed to do now.
* Fixed a bug in the backend tests to do with not using the result of an Op::Add.

* Fix the remaining tests for Arm64

* Move split loads logic into each backend

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