x86: change fetch error handling in top-level function

x86: change fetch error handling in top-level function

... and its direct helper get_sib(). Using setjmp()/longjmp() for fetch
error handling is problematic, as per
https://sourceware.org/pipermail/binutils/2023-March/126687.html. Start
using more conventional error handling instead.

Also introduce a fetch_modrm() helper, for subsequent re-use.