[cmake] 64BitTimeSupportFcntl.cpp: This file was missing...

Copied it from rom-properties.

This broke the Launchpad Ubuntu 20.04 armhf build. Not going to resubmit
it, because I don't think anyone will be using rvthtool on that OS and
platform...

(cherry picked from commit d9f4ec96f1)

cmake/macros/64BitTimeSupportFcntl.cpp

@@ -0,0 +1,19 @@
+/**
+ * 64-bit time_t test code, with fcntl.h.
+ * Reference: https://github.com/Benjamin-Dobell/Heimdall/blob/master/cmake/LargeFiles.c
+ */
+
+#include <stdint.h>
+#include <time.h>
+#include <fcntl.h>
+
+/** static_assert() macro copied from c++11-compat.h **/
+#define static_assert(expr, msg) switch (0) { case 0: case (expr): ; }
+
+int main(int argc, char *argv[])
+{
+	static_assert(sizeof(time_t) == sizeof(int64_t), "time_t is the wrong size");
+	int64_t tm64;
+	time_t tm = time(&tm64);
+	return 0;
+}