Don't expose all BSD extensions.

Just expose the ones that bionic historically leaked.

Also, many of the M_* constants in <math.h> are actually POSIX.

Change-Id: I6275df84c5866b872b71f1c8ed14e2aada12b793
diff --git a/libc/include/sys/endian.h b/libc/include/sys/endian.h
index 99d5ff1..b9e4758 100644
--- a/libc/include/sys/endian.h
+++ b/libc/include/sys/endian.h
@@ -64,7 +64,7 @@
 #define htonq(x) __swap64(x)
 #define ntohq(x) __swap64(x)
 
-#if defined(__USE_BSD)
+#if defined(__USE_BSD) || defined(__BIONIC__) /* Historically bionic exposed these. */
 #define LITTLE_ENDIAN _LITTLE_ENDIAN
 #define BIG_ENDIAN _BIG_ENDIAN
 #define PDP_ENDIAN _PDP_ENDIAN