fix x86_64 wrongly reporting itself as ILP32 instead of LP64

arch/x86_64/bits/posix.h

@@ -1,2 +1,2 @@
-#define _POSIX_V6_ILP32_OFFBIG  1
-#define _POSIX_V7_ILP32_OFFBIG  1
+#define _POSIX_V6_LP64_OFF64  1
+#define _POSIX_V6_LP64_OFF64  1