Add translation support for fragment prefixes

For asynchronous signals during detach or when a client uses an itimer, it would improve performance to fully translate registers for a PC in a fragment prefix. Xref #4232 where removing translation failures speeds up detach. Xref #7970 on translations hit in prefixes in drmemtrace.

Start by reading the context in issues #4232 and #7970, then trace the fragment-prefix translation path involved in asynchronous signals during detach and client itimers. The work is complete when registers for a PC in a fragment prefix are fully translated without the relevant translation failures and the performance impact is verified.

Written by the indexing model from the issue text.