* dcrt0.cc (init_windows_system_directory): Record system_wow64_directory

* dcrt0.cc (init_windows_system_directory): Record system_wow64_directory
information.
* exceptions.cc (_cygtls::inside_kernel): Modernize comment.  Consider
executing a DLL from the Wow64 directory as being "in the kernel".
(_cygtls::call_signal_handler): For now, only deal with main_tls signals if
main_tls is known to be executing in the cygwin DLL.  To more closely emulate
linux, consider the operation to be restartable if not executing in the main
thread.
* globals.cc (windows_system_directory): Remove NO_COPY.
(windows_system_directory_length): Ditto.
(system_wow64_directory): New variable.
(system_wow64_directory_length): Ditto.
* select.cc (cygwin_select): Don't issue a EINTR on non-main threads since that
seems to be what Linux does.  Add missing break to signal case/switch.
(select_stuff::wait): Don't issue a EINTR on non-main threads since that seems
to be what Linux does.  Remove now-unneeded accommodation for
WAIT_IO_COMPLETION.  Add a comment.
* sigproc.h (cygwait): Ditto.  Don't return if signal_received noticed and it's
not the main thread.
* signal.cc (sigprocmask): Add standard syscall debug stuff.
* thread.cc (pthread_sigmask): Ditto.