Add some extensions to pthread for armv6k-nintendo-3ds

[AzureMarker]

The pthread_attr_t type can have priority and affinity values set.
pthread_getpriority returns the priority of the current thread.

These are needed to enable std thread support. std can't link directly with libctru so we go through pthread as an intermediate interface.

[Meziu]

No, we shouldn’t define functions not normally found in standard declarations.
The pthread_attr_t type has 56 bytes of available memory, can’t we use pointer math on that instead? We could put default values when initializing the struct (in pthread_attr_init) and otherwise change them via pointer math or an array with as in the std directly. It doesn’t look good, but at least we won’t be breaking this rule.

Default values could be: the spawning thread’s priority, and the normal core (0).

[AzureMarker]

That sounds really cursed and I doubt std will merge that. That's tying std even closer to the implementation details.

There's other pthread functions only found on certain platforms in the libc crate already, so I think this is somewhat normal.

[Meziu]

I really don’t like both ideas honestly… let’s go with this one anyways. It isn’t directly tied to devkitPRO, but we are putting ourselves in a weird position. Let’s go on with this for now.