wakeup_write: make sure the eventfd write sends eight bytes

wakeup_write: make sure the eventfd write sends eight bytes

The eventfd manpage says:

  A write(2) fails with the error EINVAL if the size of the supplied
  buffer is less than 8 bytes

When doing x32 on a 64-bit system, pointers are still four bytes so this
code must not use the size of a pointer but the size of a 64-bit type.

Fixes #16237
Reported-by: Jan Engelhardt
Closes #16239