Demo/Posix_GCC: add demo application for Posix port using GCC

This is largely a copy of the Windows demo application with a few key
changes:

- heap_3 (use malloc()/free()) so tools like valgrind "just work".

- printf() wrapped in a mutex to prevent deadlocks on the internal
  pthread mutexes inside printf().

SCons (https://scons.org/) is used as the build system.

This will be built as a 64-bit application, but note that the memory
allocation trace points only record the lower 32-bits of the address.

FreeRTOS/Demo/Posix_GCC/src/console.h

@@ -0,0 +1,45 @@
+/*
+ * FreeRTOS Kernel V10.3.0
+ * Copyright (C) 2020 Cambridge Consultants Ltd.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * http://www.FreeRTOS.org
+ * http://aws.amazon.com/freertos
+ *
+ * 1 tab == 4 spaces!
+ */
+#ifndef CONSOLE_H
+#define CONSOLE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*-----------------------------------------------------------
+ * Example console I/O wrappers.
+ *----------------------------------------------------------*/
+
+void console_init(void);
+void console_print(const char *fmt, ...);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CONSOLE_H */