More <limits.h> fixes.

Went through the POSIX spec for the _POSIX* and _XOPEN* constants.

Bug: http://b/32776472
Test: ran tests
Change-Id: I389100dbc7de354eae9056e44b0a7fa8c37374e3
diff --git a/libc/include/limits.h b/libc/include/limits.h
index 4384f6f..48e7ea9 100644
--- a/libc/include/limits.h
+++ b/libc/include/limits.h
@@ -61,6 +61,7 @@
 #else
 # define LONG_BIT 32
 #endif
+#define WORD_BIT 32
 
 #define	SCHAR_MAX	0x7f		/* max value for a signed char */
 #define SCHAR_MIN	(-0x7f-1)	/* min value for a signed char */
@@ -130,6 +131,8 @@
 
 #define MB_LEN_MAX 4
 
+#define NZERO 20
+
 #define IOV_MAX 1024
 #define SEM_VALUE_MAX 0x3fffffff