linker/PEi386: Don't sign-extend symbol section number

Previously we incorrectly interpreted PE section numbers as signed
values. However, this isn't the case; rather, it's an unsigned 16-bit number
with a few special bit-patterns (0xffff and 0xfffe). This resulted in #22941
as the linker would conclude that the sections were invalid.

Fixing this required quite a bit of refactoring.

Closes #22941.