signal.c: fix stack overflow check on Mac OS X

* signal.c (check_stack_overflow): fix condition to use ucontext
  register, mcontext_t dereference, and its member names, on Mac
  OS X.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46100 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vm_dump.c: adjust style

* vm_dump.c (backtrace): adjust style, inserting breaks between
  type, function name with its arguments, and its body.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46101 b2dd03c8-39d4-4d8f-98ff-823fe69b080e