add API routine to create a basic block without executing it

There are some use cases that involve creating a basic block but not executing it. This has some potential issues with DR assumptions on execution (current isa mode, mcontext state at top of bb, trace building, lazy linking) but I do not think there are any roadblocks.

The issue names no files or tests, only a proposed API routine for creating a basic block without executing it. Start by locating existing basic-block creation and execution entry points, then examine the stated assumptions around ISA mode, mcontext state, trace building, and lazy linking. Done would require a settled API design and validation of the non-executing path.

Written by the indexing model from the issue text.