define pw_gecos to pw_passwd for LP32 systems

Since we set both fields to NULL all the time, we can alias the two
fields together on LP32 to provide build-time compatibility.

BUG=24918750
TEST=python now builds for Brillo boards

Change-Id: I3394aea80c9a5288669389f71d0e4132f2157e3c
diff --git a/libc/include/pwd.h b/libc/include/pwd.h
index 6012b96..905bc75 100644
--- a/libc/include/pwd.h
+++ b/libc/include/pwd.h
@@ -106,6 +106,9 @@
   gid_t pw_gid;
 #ifdef __LP64__
   char* pw_gecos;
+#else
+  // Note: On LP32, we define pw_gecos to pw_passwd since they're both NULL.
+# define pw_gecos pw_passwd
 #endif
   char* pw_dir;
   char* pw_shell;