Preserve return registers for unhandled Penguin hypercalls

[lacraig2]

Summary

• Preserve guest return registers when a Penguin TCG marker is not handled by the registered callback.
• Match old PANDA semantics: unhandled marker callbacks do not modify guest-visible return registers.
• Keep MIPS preserving nr/$v0; preserve a0/return register for PPC, ARM/AArch64, RISC-V, and LoongArch.

Validation

• git diff --check
• Earlier isolated MIPS validation with patched libqemu-system-mipseb.so in the Penguin PR image:
  • mipseb basic target, Linux 4.10: 5/5 passes
  • mipsel basic target, Linux 4.10: 1/1 pass