The monolithic rules, largely but not entirely identical between ports,
were pretty ugly to fiddle with. Break them up, and use (largely) the
same rules for all ports (x86'es xen.efi being somewhat special, though).

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

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: RISC-V: place .sdata / .srodata / .riscv.attributes

Jan