x86/COFF: support section-index relocations in insn operands

x86/COFF: support section-index relocations in insn operands

On the grounds of the principle put down near the bottom of [1], along
with image and section relative operations, let's also support as insn
operands what .secidx is for on the data side (of course like elsewhere
the reloc operator can then also be used for data generation, albeit a
small tweak to x86_cons() is needed for this to work).

[1] https://sourceware.org/pipermail/binutils/2024-November/137617.html