posix: pthread: embed posix_thread_attr inside of posix_thread

Rather than shadowing most of the fields from pthread_attr_t
(struct posix_thread_attr) inside of struct posix_thread, just
embed the structure itself.

Additionally, use pthread_attr_init() to allocate a thread
stack and pthread_attr_destroy() to destroy a thread stack,
dramatically simplifying pthread_create().

A fair bit of rework was needed already to mitigate bitrot
issues in the testsuite. More should probably be done later.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>
4 files changed