compiler.h: unify system ifdef cruft here

Shove a lot of the HOSTCC and related #ifdef checking crap into the new
compiler.h header so that we can keep all other headers nice and clean.

Also introduce custom uswap functions so we don't have to rely on the non
standard implementations that a host may (or may not in the case of OS X)
provide.  This allows mkimage to finally build cleanly on an OS X system.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/tools/os_support.h b/tools/os_support.h
index f6f86b0..7bf930e 100644
--- a/tools/os_support.h
+++ b/tools/os_support.h
@@ -19,6 +19,8 @@
 #ifndef __OS_SUPPORT_H_
 #define __OS_SUPPORT_H_
 
+#include "compiler.h"
+
 /*
  * Include additional files required for supporting different operating systems
  */