[libc] Update prctl() declaration to use variadic arguments. (#198654)

prctl declaration should typically use variadic arguments (e.g. see
https://man7.org/linux/man-pages/man2/prctl.2.html), as the types /
quantity of subsequent arguments depends on the `option`. We can't
depend on all `<prctl.h>` users to explicitly cast arguments to
`unsigned long` and passing all 5 of them every time.

* Don't add any option-specific logic, and just consume `arg2`-`arg5`
from variadic arguments and pass them to syscall implementation as-is,
assuming that they won't be used by the kernel if they are not needed,
and consuming these arguments won't lead to crashes.
* Updated the test to use `prctl` variants with less than 5 explicit
arguments (for PR_SET_NAME and PR_GET_NAME).

GitOrigin-RevId: 57d2c4c7889333f8d1ebb77efdb5195c2e4f5e1e