The monolithic rules, largely but not entirely identical between ports,
were pretty ugly to fiddle with. They also ended up going out of sync
when really they would better have stayed consistent. Break them up,
and use (largely) the same rules for all ports (x86'es xen.efi being
somewhat special, though).

The final three patches are related only in so far as they address
observations made while doing the conversion.

v3 addresses review feedback (patch 1 only; see there for details).

1: x86: split xen-syms/xen.efi linking rules
2: Arm: split xen-syms linking rule
3: RISC-V: split xen-syms linking rule
4: PPC: split xen-syms linking rule
5: build: move $(all-symbols-*)
6: build: move $(compare-symbol-tables)

Jan