CMake: Don't use __builtin_unreachable() with GCC 9.0-9.4 and 10.0-10.3

These compilers are known to miscompile the save loading code due to
a bug in the variable range information gained from these hints.
This bug was fixed in GCC 11 and has now been backported to the 9 and
10 branches.

See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97953
Fixes: issue #1600
(cherry picked from commit c134113)

CMakeLists.txt

@@ -721,7 +721,13 @@ else(MSVC)
 	endif()
 	check_builtin(ARX_HAVE_BUILTIN_ASSUME_ALIGNED   "__builtin_assume_aligned")
 	check_builtin(ARX_HAVE_BUILTIN_TRAP             "__builtin_trap")
-	check_builtin(ARX_HAVE_BUILTIN_UNREACHABLE      "__builtin_unreachable")
+	if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND
+	   ((NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9.5") OR
+	    (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "10" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "10.4")))
+		message(STATUS "Not using __builtin_unreachable() with unpatched GCC 9/10 due to a compiler bug https://arx.vg/1600")
+	else()
+		check_builtin(ARX_HAVE_BUILTIN_UNREACHABLE      "__builtin_unreachable")
+	endif()
 	if(NOT ARX_HAVE_CXX11_ALIGNOF)
 		check_builtin(ARX_HAVE_GCC_ALIGNOF            "__alignof__")
 	endif()