merge revision(s) r45517,r45544,r45924:

	signal.c: check stack overflow by SP

	* signal.c (check_stack_overflow): raise SystemStackError if SP
  register and fault address is in the same page, on x86 linux.
  [EXPERIMENTAL]
	* signal.c (check_stack_overflow): Don't use ucontext_t if ucontext.h
	  is not available.
	  Fixes build on Android (x86).


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e