x86/syscalls: Make __X32_SYSCALL_BIT be unsigned long

Currently, it's an int.  This is bizarre.  Fortunately, the code using it
still works: ~__X32_SYSCALL_BIT is also int, so, if nr is unsigned long,
then C kindly sign-extends the ~__X32_SYSCALL_BIT part, and it actually
results in the desired value.

This is far more subtle than it deserves to be.  Syscall numbers are, for
all practical purposes, unsigned long, so make __X32_SYSCALL_BIT be
unsigned long.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/99b0d83ad891c67105470a1a6b63243fd63a5061.1562185330.git.luto@kernel.org