Fix x86 gdbserver build.

Add the x86-only elf_fpxregset_t to <sys/procfs.h>, and rename
user_fxsr_struct (the kernel's name) to user_fpxregs_struct (the
userspace name).

Bug: http://b/22068064
Change-Id: Id466096bf02baf6bd49b193f49d9036848d33548
diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index b370add..d63fe6a 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -47,7 +47,7 @@
   long fos;
   long st_space[20];
 };
-struct user_fxsr_struct {
+struct user_fpxregs_struct {
   unsigned short cwd;
   unsigned short swd;
   unsigned short twd;