Cygwin: main exception handler (64-bit): continue GCC exceptions

This is necessary in order to be consistent with the following comment
in the definition of _Unwind_RaiseException() in the GCC source file
libgcc/unwind-seh.c:

     The exception handler installed in crt0 will continue any GCC
     exception that reaches there (and isn't marked non-continuable).

Previously we failed to do this and, as a consequence, the C++ runtime
didn't call std::terminate after an unhandled exception.

This fixes the problem reported here:

  https://cygwin.com/pipermail/cygwin/2019-October/242795.html
  https://sourceware.org/pipermail/cygwin/2020-August/245897.html

winsup/cygwin/exceptions.cc

@@ -762,6 +762,25 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in,
 	 handling.  */
       return ExceptionContinueExecution;
 
+#ifdef __x86_64__
+/* From the GCC source file libgcc/unwind-seh.c. */
+#define STATUS_USER_DEFINED		(1U << 29)
+#define GCC_MAGIC			(('G' << 16) | ('C' << 8) | 'C')
+#define GCC_EXCEPTION(TYPE)		\
+       (STATUS_USER_DEFINED | ((TYPE) << 24) | GCC_MAGIC)
+#define STATUS_GCC_THROW		GCC_EXCEPTION (0)
+#define STATUS_GCC_UNWIND		GCC_EXCEPTION (1)
+#define STATUS_GCC_FORCED		GCC_EXCEPTION (2)
+
+    case STATUS_GCC_THROW:
+    case STATUS_GCC_UNWIND:
+    case STATUS_GCC_FORCED:
+      /* According to a comment in the GCC function
+	 _Unwind_RaiseException(), GCC expects us to continue all the
+	 (continuable) GCC exceptions that reach us. */
+      return ExceptionContinueExecution;
+#endif
+
     default:
       /* If we don't recognize the exception, we have to assume that
 	 we are doing structured exception handling, and we let

winsup/cygwin/release/3.1.7

@@ -11,3 +11,7 @@ Bug Fixes:
 
 - Fix an mmap issue that could cause failure with errno EFBIG
   Partially addresses: https://sourceware.org/pipermail/cygwin/2020-July/245557.html
+
+- Fix the behavior of C++ apps after an unhandled exception (64-bit only)
+  Addresses: https://cygwin.com/pipermail/cygwin/2019-October/242795.html
+             https://cygwin.com/pipermail/cygwin/2020-August/245897.html