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/wincap.cc

@@ -21,6 +21,7 @@ details. */
    puzzled that this has never been noticed before... */
 
 wincaps wincap_xpsp2 __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:1,
   max_sys_priv:SE_CREATE_GLOBAL_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:false,
@@ -46,9 +47,11 @@ wincaps wincap_xpsp2 __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:false,
   has_precise_system_time:false,
   has_microsoft_accounts:false,
+  has_set_thread_stack_guarantee:false,
 };
 
 wincaps wincap_2003 __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:1,
   max_sys_priv:SE_CREATE_GLOBAL_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:false,
@@ -74,9 +77,11 @@ wincaps wincap_2003 __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:false,
   has_precise_system_time:false,
   has_microsoft_accounts:false,
+  has_set_thread_stack_guarantee:true,
 };
 
 wincaps wincap_vista __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:1,
   max_sys_priv:SE_CREATE_SYMBOLIC_LINK_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:true,
@@ -102,9 +107,11 @@ wincaps wincap_vista __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:true,
   has_precise_system_time:false,
   has_microsoft_accounts:false,
+  has_set_thread_stack_guarantee:true,
 };
 
 wincaps wincap_7 __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:1,
   max_sys_priv:SE_CREATE_SYMBOLIC_LINK_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:true,
@@ -130,9 +137,11 @@ wincaps wincap_7 __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:true,
   has_precise_system_time:false,
   has_microsoft_accounts:false,
+  has_set_thread_stack_guarantee:true,
 };
 
 wincaps wincap_8 __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:2,
   max_sys_priv:SE_CREATE_SYMBOLIC_LINK_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:true,
@@ -158,9 +167,11 @@ wincaps wincap_8 __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:true,
   has_precise_system_time:true,
   has_microsoft_accounts:true,
+  has_set_thread_stack_guarantee:true,
 };
 
 wincaps wincap_10 __attribute__((section (".cygwin_dll_common"), shared)) = {
+  def_guard_pages:2,
   max_sys_priv:SE_CREATE_SYMBOLIC_LINK_PRIVILEGE,
   is_server:false,
   has_mandatory_integrity_control:true,
@@ -186,6 +197,7 @@ wincaps wincap_10 __attribute__((section (".cygwin_dll_common"), shared)) = {
   terminate_thread_frees_stack:true,
   has_precise_system_time:true,
   has_microsoft_accounts:true,
+  has_set_thread_stack_guarantee:true,
 };
 
 wincapc wincap __attribute__((section (".cygwin_dll_common"), shared));
@@ -240,6 +252,8 @@ wincapc::init ()
   ((wincaps *)caps)->is_server = (version.wProductType != VER_NT_WORKSTATION);
 #ifdef __x86_64__
   wow64 = 0;
+  /* 64 bit systems have one more guard page than their 32 bit counterpart. */
+  ++((wincaps *)caps)->def_guard_pages;
 #else
   if (NT_SUCCESS (NtQueryInformationProcess (NtCurrentProcess (),
 					     ProcessWow64Information,