make pthread attribute types structs, even when they just have one field

this change is to get the right tags for C++ ABI matching. it should
have no other effects.

src/thread/pthread_attr_get.c

@@ -52,19 +52,19 @@ int pthread_attr_getstacksize(const pthread_attr_t *restrict a, size_t *restrict
 
 int pthread_barrierattr_getpshared(const pthread_barrierattr_t *restrict a, int *restrict pshared)
 {
-	*pshared = !!*a;
+	*pshared = !!a->__attr;
 	return 0;
 }
 
 int pthread_condattr_getclock(const pthread_condattr_t *restrict a, clockid_t *restrict clk)
 {
-	*clk = *a & 0x7fffffff;
+	*clk = a->__attr & 0x7fffffff;
 	return 0;
 }
 
 int pthread_condattr_getpshared(const pthread_condattr_t *restrict a, int *restrict pshared)
 {
-	*pshared = *a>>31;
+	*pshared = a->__attr>>31;
 	return 0;
 }
 
@@ -75,24 +75,24 @@ int pthread_mutexattr_getprotocol(const pthread_mutexattr_t *restrict a, int *re
 }
 int pthread_mutexattr_getpshared(const pthread_mutexattr_t *restrict a, int *restrict pshared)
 {
-	*pshared = *a>>31;
+	*pshared = a->__attr>>31;
 	return 0;
 }
 
 int pthread_mutexattr_getrobust(const pthread_mutexattr_t *restrict a, int *restrict robust)
 {
-	*robust = *a / 4U % 2;
+	*robust = a->__attr / 4U % 2;
 	return 0;
 }
 
 int pthread_mutexattr_gettype(const pthread_mutexattr_t *restrict a, int *restrict type)
 {
-	*type = *a & 3;
+	*type = a->__attr & 3;
 	return 0;
 }
 
 int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t *restrict a, int *restrict pshared)
 {
-	*pshared = *(int *)a;
+	*pshared = a->__attr[0];
 	return 0;
 }