Handle the GAS oddity for compatability with `gcc -S` output.

[pharaun]

Basically these are the following oddity that I've noticed in GAS so far. (The description might not be 100% accurate)

• LUI takes the lower 20 bits, not the upper 20 bit, and uses %LOW and %HI() to shift the bits (HI = high 20bit shifted over 12) (low = grabbing the low 12 bits)
• Ordering in sw in GAS is swapped
• GAS syntax such as:

//      - add x0, x1, x3
//      - lw x0, 0x0(x3)
//      - sw x0, 0x0(x3)
//      - csr{rw, rs, rc} a0, cycle, x0
//      - csr{rw, rs, rc}i a1, sscratch, 1

Look into if I want to have 1:1 compat with GAS in the parser/assembler and have a flag to enable/disable the compat, or if it would be better to just provide a transpiler of sorts that can take the GAS syntax outputted by gcc -S and reformat it into a format that my assembler can take.