error.c: rb_get_backtrace

	* error.c (rb_get_backtrace): move from eval_error.c to call
  exc_backtrace directly.  [ruby-core:78097] [Bug #12925]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@56781 b2dd03c8-39d4-4d8f-98ff-823fe69b080e