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

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.

Author: AzureMarker

src/unix/newlib/horizon/mod.rs

@@ -190,5 +190,11 @@ extern "C" {
         value: *mut ::c_void,
     ) -> ::c_int;
 
+    pub fn pthread_attr_setpriority(attr: *mut ::pthread_attr_t, priority: ::c_int) -> ::c_int;
+
+    pub fn pthread_attr_setaffinity(attr: *mut ::pthread_attr_t, affinity: ::c_int) -> ::c_int;
+
+    pub fn pthread_getpriority() -> ::c_int;
+
     pub fn gethostid() -> ::c_long;
 }