Move the huge page size defines into arch code

It's 2MB on x86-64 by default, and 4MB on x86.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os.h b/os.h
index 85db5b8..3f83905 100644
--- a/os.h
+++ b/os.h
@@ -51,8 +51,10 @@
 #define SHM_HUGETLB			0
 #define FIO_HUGE_PAGE			0
 #else
+#ifndef FIO_HUGE_PAGE
 #define FIO_HUGE_PAGE			4194304
 #endif
+#endif
 
 #if __GNUC__ < 3
 #define __must_check