Fix original stack when running signal handler on alternate stack

* autoload.cc (SetThreadStackGuarantee): Import.
        * cygtls.h (struct _cygtls): Replace thread_context with a ucontext_t
        called context.
        * exceptions.cc (exception::handle): Exit from process via signal_exit
        in case sig_send returns from handling a stack overflow SIGSEGV.
        Explain why.
        (dumpstack_overflow_wrapper): Thread wrapper to create a stackdump
        from another thread.
        (signal_exit): Fix argument list to reflect three-arg signal handler.
        In case we have to create a stackdump for a stack overflow condition,
        do so from a separate thread.  Explain why.
        (sigpacket::process): Don't run signal_exit on alternate stack.
        (altstack_wrapper): Wrapper function to do stack correction when
        calling the signal handler on an alternate stack to handle a stack
        overflow.  Make sure to have lots of comments.
        (_cygtls::call_signal_handler): Drop local context variable to reduce
        stack pressure.  Use this->context instead.  Change inline assembler
        to call altstack_wrapper.
        (_cygtls::signal_debugger): Accommodate aforementioned change to
        struct _cygtls.
        * tlsoffset.h: Regenerate.
        * tlsoffset64.h: Regenerate.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/cygtls.h

@@ -17,6 +17,7 @@ details. */
 #include <mntent.h>
 #undef _NOMNTENT_FUNCS
 #include <setjmp.h>
+#include <ucontext.h>
 
 #define CYGTLS_INITIALIZED 0xc763173f
 
@@ -192,7 +193,8 @@ public:
   siginfo_t *sigwait_info;
   HANDLE signal_arrived;
   bool will_wait_for_signal;
-  CONTEXT thread_context;
+  long __align;			/* Needed to align context to 16 byte. */
+  ucontext_t context;
   DWORD thread_id;
   siginfo_t infodata;
   struct pthread *tid;