Avoid requiring the bit number of bit, set, and res to be constant

[ISSOtm]

It is feasible to defer computation of the opcode byte to the linker.

[Rangi42]

Was this motivated by some particular project's example that could benefit from link-time bit indexes?

[Rangi42]

Oh, I see, https://github.com/ISSOtm/gb-vwf/blob/e850d8431a868ca738002b2ee0b2b0ade9c63d71/vwf.asm#L212-L214:

; Definitions for the various bits in `wFlags`.
; FIXME: it would be neater to define these next to `wFlags`, but RGBDS 0.6.1 requires the first
; operand to `bit`, `set`, and `res` to be constant...