Magic numbers in `taint.c/taintprop()`

[phillipstanleymarbell]

Magic numbers in taint.c/taintprop() magic numbers (unnamed constants) should be replaced with the named constants in decode-riscv.h. @HAM41 or @samuelmswong, interested in looking at this?

[SamuelmsWong-zz]

Will start on this on Thurs if no-one's touched it by then.

[HAM41]

I should be able to have a look at this on Wednesday. If this is about TAINTMEMBASE or similar then those are based on the magic numbers for normal MEMBASE