PR4186: cross-architecture probe building

PR4186: cross-architecture probe building

* main.cxx (main): Add 'a:' and 'B:' options.
* session.h (kbuildflags): New place to store -B args.
* testsuite/systemtap.base/cmd_parse.exp: Test them lightly.
* buildrun.cxx (run_make_cmd): Use "--no-print-directory"
  rather than ">/dev/null" in kbuild invocations.  Pass
  '-a' and '-B' flags along.
* hash.cxx (find_script_hash): Add them.
* NEWS, stap.1.in: Mention this.