Add real mode x86 decoding and context reset

In-progress work checkpointed on the pbr branch; not ready for review
or merge.

Gate the x86 SLEIGH definitions on protectedMode so 16-bit real mode
decodes correctly, and set that context variable in the existing x86
compiler specs. Split the native Context into a shared ContextState so
a context can be reset without invalidating translations that still
reference its address spaces.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: zardus