Debugger: Call frame unwind optimizations.

- Pass address size to DwarfManager, and subsequently DwarfFile's Load()
  method, as that information is needed in order to parse the frame
  information correctly early on.
- When initially loading debug information, also do a quick pass through
  .{debug,eh}_frame, and build a lookup table of offsets. This is then
  used later when actual unwinding is requested in order to quickly find
  the corresponding FDE/CIE. Should noticeably improve performance when
  stepping through code, especially for larger/more complex executable images.

Implements #8613.