glib/gthread-posix: Conditionally use `futex` and/or `futex_time64` syscalls as necessary and use the correct `struct timespec` definition

On some systems only `futex_time64` exists (e.g. riscv32) while on
others only `futex` exists (old Linux, 64 bit platforms), so it is
necessary to check for both and try calling both at runtime.

Additionally use the correct `struct timespec` definition. There is not
necessarily any relation between the libc's definition and the kernel's.

Specifically, the libc headers might use 64-bit `time_t` while the kernel
headers use 32-bit `__kernel_old_time_t` on certain systems.

To get around this problem we
  a) check if `futex_time64` is available, which only exists on 32-bit
     platforms and always uses 64-bit `time_t`.
  b) otherwise (or if that returns `ENOSYS`), we call the normal `futex`
     syscall with the `struct timespec` used by the kernel, which uses
     `__kernel_long_t` for both its fields. We use that instead of
     `__kernel_old_time_t` because it is eq...