posix: headers: harmonize remaining types with newlib

The remaining types that needed to be harmonized between
Newlib and Zephyr's POSIX definitions are:

* `struct sched_param`
  - don't re-define if using minimal libc
* `pthread_attr_t`
  - convert to `struct pthread_attr`
  - define type if using minimal libc
  - assert acceptible object size
* `pthread_mutexattr_t`
  - convert to `struct pthread_mutexattr`
  - define type if using minimal libc
  - assert acceptible object size
* `pthred_condattr_t`
  - convert to `struct pthread_condattr`
  - define type if using minimal libc
  - assert acceptible object size
* `pthread_once_t`
  - adopt newlib definition
  - define type if using minimal libc
Signed-off-by: Chris Friedt <cfriedt@meta.com>