Fix x86_64 user_fpregs_struct to use the same name for the tag word as glibc.

Glibc calls the tag pointer in user_fpregs_struct ftw instead of twd.

Change-Id: I4a8dab777fa889de0b9927035cd18557ae6f3263
diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index 21803d8..18684f1 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -103,7 +103,7 @@
 struct user_fpregs_struct {
   unsigned short cwd;
   unsigned short swd;
-  unsigned short twd;
+  unsigned short ftw;
   unsigned short fop;
   __u64 rip;
   __u64 rdp;