arch: added missing MIPS HI/LO register defs

[Tazdevil971]

As per title, this pull request adds missing HI/LO register definitions, those registers are used in older MIPS CPU (pre R6).

Register DWARF numbers come directly from LLVM

[philipc]

The rest of the MIPS register definitions have a $ prefix. I assume these should too.

[Tazdevil971]

Thanks for the speedy reply! I updated the patch to now include the $ prefix. My original thought was that since the HI/LO registers are special, and cannot be referenced or named directly in assembly (they can only be accessed though special mflo/mfhi/mtlo/mthi instructions), it would have been better to separate them from the rest.

But looking at other registers definitions no one does that, also I agree that it looks better like this.

[philipc]

Thanks!