Adds a new extension "drcallstack" which provides callstack walking
facilities.  This initial implementation adds a libunwind-based
implementation and targets only Linux for now.

Adds an interface to walk one step at a time over callstack frames.
The implementation converts the dr_mcontext_t into libunwind's context
structure and invokes the libunwind step API.

Getting libunwind to work requires several steps:

+ Ignore libpthread exports when importing any symbol that does not start with
  "pthread".  Otherwise libunwind crashes using __errno_location from libpthread
  instead of from libc.  We add a warning when a private libpthread is loaded
  to help diagnose any other potential problems (xref #956).

+ Have dl_iterate_phdr operate on app libraries instead of private libraries.
  This is done with a new flag and logic in the redirection code, with the
  flag set for libraries named "libunwind*".

A statically-linked libunwind is not supported.

Updates drwrap to set the mcontext pc field to simplify usage.

Adds a test and documentation.

Adds a sample client showing how to use this library.

Installs libunwind for cross-compiling and for 32-bit x86 using manual download and unpack steps, as there are no ready-made packages (GA CI uses a microsoft.com repository which has only 64-bit libunwind, and file conflicts are hit when trying to add a new repository for 32-bit only).

Issue: #2414, #956