Minor refactoring/cleanup in gdb tools

Summary: Nothing to see here.

Reviewed By: @edwinsmith

Differential Revision: D1491456
Add some lookup commands for GDB

Summary: Create two new GDB prefix commands:
  - lookup func <FuncId>
  - lookup litstr <Id> [Unit*]

They lookup the expected object and calls the print command on them (so
that $ and $$ and ${#} work as expected).

Also add a `unit' command which sets the "current Unit", used by various
commands (like `lookup litstr' and `hhx').

Finally, improve printing in `hhx':
  - literal strings for SA immediates instead of IDs
  - print short ints as ints only, instead of both ints and chars
  - remove the <> around immediates
  - make output format more like `x'

Reviewed By: @edwinsmith

Differential Revision: D1491458
Port walkstk into GDB Python bindings

Summary: This is a hybrid of @markw65's `walkstk` and `walkstk2` commands.  It
walks the $rbp chain, printing out a backtrace including both native and
VM frames and functions.  The output format is briefer, and does not
contain the instruction found at $rip.

Reviewed By: @markw65

Differential Revision: D1491681