Support RPN operand $62 BIT_INDEX for link-time bit/res/set

[Rangi42]

Implemented for RGBDS 0.9.2 in gbdev/rgbds#1654

This assembly:

section "test", rom0
bit VALUE, a
res VALUE * 2 + 3, b
set VALUE * VALUE, [hl]

Gives this output for rgbobj -p rpn:

SECTION "test"
    RPN expression:
        Sym#0 Sym#0 * SET<[HL]>
    RPN expression:
        Sym#0 $0002 * $0003 + RES<B>
    RPN expression:
        Sym#0 BIT<A>

And this output for rgbobj -p rpn -r infix:

SECTION "test"
    infix expression:
        SET<[HL]> (Sym#0 * Sym#0)
    infix expression:
        RES<B> (Sym#0 * $0002 + $0003)
    infix expression:
        BIT<A> Sym#0

[ISSOtm]

Thank you!