the omission of the padding was uncovered by the latest regression
statvfs regression test added to libc-test.

the definition was found to be incorrect at least for powerpc, and
fixing this cleanly requires making the definition arch-specific. this
will allow cleaning up the definition for other archs to make it more
specific, and reversing some of the ugliness (time_t hacks) introduced
with the x32 port.

this first commit simply copies the existing definition to each arch
without any changes. this is intentional, to make it easier to review
changes made on a per-arch basis.

the kernel uses long longs in the struct, but the documentation
says they're long. so we need to fixup the mismatch between the
userspace and kernelspace structs.
since the struct offers a mem_unit member, we can avoid truncation
by adjusting that value.

previously it was wrongly using the x86_64 one, precluding having both
x32 and x86_64 libs present on the same system.

it's legal to call the __syscall functions with more arguments than
necessary, and the __syscall_cp cancel dummy impl. does just that.

thus we must insert the switch for all possible syscalls numbers
into all of the syscallN inline functions.

- the nanosleep fixup "fixed" the second timespec* argument erroneusly.
- the futex fixup was missing the check for FUTEX_WAIT.
- general cleanup using a macro.