Define x86_64 context.

diff --git a/include/basetsd.h b/include/basetsd.h
index 987d843..d9db9b8 100644
--- a/include/basetsd.h
+++ b/include/basetsd.h
@@ -149,6 +149,10 @@
 # undef  WORDS_BIGENDIAN
 # undef  BITFIELDS_BIGENDIAN
 # define ALLOW_UNALIGNED_ACCESS
+#elif defined(__x86_64__)
+# undef  WORDS_BIGENDIAN
+# undef  BITFIELDS_BIGENDIAN
+# define ALLOW_UNALIGNED_ACCESS
 #elif defined(__sparc__)
 # define WORDS_BIGENDIAN
 # define BITFIELDS_BIGENDIAN