Introduce alternate_signal_stack RAII class
    
    This introduces a new RAII class that temporarily installs an
    alternate signal stack (on systems that have sigaltstack); then
    changes the one gdb use of sigaltstack to use this class instead.
    
    This will be used in a later patch, when creating new threads that may
    want to handle SIGSEGV.
    
    gdb/ChangeLog
    2019-09-30  Tom Tromey  <tom@tromey.com>
    
    	* main.c (setup_alternate_signal_stack): Remove.
    	(captured_main_1): Use gdb::alternate_signal_stack.
    	* gdbsupport/alt-stack.h: New file.