In sysdeps/unix/sysv/linux/x86_64/sys/user.h when __WORDSIZE==64, the struct
user_fpregs_struct is defined using __uintXX_t types. These are not defined in
any of its #include's so any program that includes <sys/user.h> without
including <bits/types.h> first leads to a compile failure on x86_64.
I realise that sys/user.h says "Don't use it for anything other than GDB unless
you know what you are doing", but I am unsure whether requiring an additional
include on one particular platform is covered by that...
I've checked in a patch but you shouldn't use the header.