Add maiko/platform.h to detect OS + CPU. (#216)

This also adds a bit to the `-info` output to say which OS and CPU
are being targeted.

This can be used in subsequent commits to drive whether or not
we need `BYTESWAP` and whether or not unaligned memory access
is okay.

We'll be able to remove per-platform defines from all of the
makefiles and cmake.

src/main.c

@@ -736,6 +736,7 @@ void print_info_lines() {
 #elif (RELEASE == 351)
   printf("Emulator for Medley release 3.51\n");
 #endif /* RELEASE */
+  printf("Compiled for %s (%s) (%d bit).\n", MAIKO_OS_NAME, MAIKO_ARCH_NAME, MAIKO_ARCH_WORD_BITS);
   printf("Creation date: %s", ctime(&MDate));
 #ifdef LPSOLVE
   printf("Contains lp_solve LP solver.\n");