Improve process exit status macros on MinGW

Improve process exit status macros on MinGW

When a Windows program is terminated by a fatal exception, its exit
code is the value of that exception, as defined by the various
EXCEPTION_* symbols in the Windows API headers.  This commit emulates
WTERMSIG etc. by translating the fatal exception codes to more-or-less
equivalent Posix signals.

gdb/ChangeLog:
2020-01-06  Eli Zaretskii  <eliz@gnu.org>
	    Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_SFILES): Add gdbsupport/gdb_wait.c.
	* windows-tdep.c: New enumeration of WINDOWS_SIG* signals.
	(windows_gdb_signal_to_target): New function, uses the above
	enumeration to convert GDB internal signal codes to equivalent
	Windows codes.
	(windows_init_abi): Call set_gdbarch_gdb_signal_to_target.
	* windows-nat.c: Include "gdb_wait.h".
	(get_windows_debug_event): Extract the fatal exception from the
	exit status and convert to the equivalent Posix signal number.
	* cli/cli-cmds.c (exit_status_set_internal_vars): Account for the
	possibility that WTERMSIG returns GDB_SIGNAL_UNKNOWN.
	* gdbsupport/gdb_wait.c: New file, implements
	windows_status_to_termsig.
	* gdbsupport/gdb_wait.h (WIFEXITED, WIFSIGNALED, WEXITSTATUS)
	(WTERMSIG) [__MINGW32__]: Separate definitions for MinGW.

gdb/gdbserver/ChangeLog:
2020-01-06  Eli Zaretskii  <eliz@gnu.org>
	    Pedro Alves  <palves@redhat.com>

	* win32-low.c (get_child_debug_event): Extract the fatal exception
	from the exit status and convert to the equivalent Posix signal
	number.
	(win32_wait): Allow TARGET_WAITKIND_SIGNALLED status as well.
	* Makefile.in (OBS, SFILES): Add gdb_wait.[co].