report bug with machine regisiters

* error.c (rb_bug_context): new function to report bug with
  context.
* vm_dump.c (rb_vm_bugreport): accepts `ucontext_t` argument to
  dump machine regisiters.  based on [GH-584].
* signal.c (sigbus, sigsegv): dump machine regisiters if available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46106 b2dd03c8-39d4-4d8f-98ff-823fe69b080e