Properly stub out pthread cancellation

Although it is not supported in WASI, thread cancellation is an important
feature of pthreads with wide-ranging implications.

Provide the best stubs we can within the limitations by allowing
cancellation type and state to be set, even though they will have no
effect as pthread_cancel is stubbed to always return ENOTSUP

Remove the THREAD_MODEL=single guard as it's currently not being
compiled in that case, and because we intend to provide
single-threaded stub implementations in the future.
Incidentally replace the existing pthread_setcancelstate stub
with one which actually correctly writes a value to the old parameter.

Makefile

@@ -300,6 +300,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/pthread_barrierattr_init.c \
         thread/pthread_barrierattr_setpshared.c \
         thread/pthread_cleanup_push.c \
+        thread/pthread_cancel.c \
         thread/pthread_cond_broadcast.c \
         thread/pthread_cond_destroy.c \
         thread/pthread_cond_init.c \
@@ -345,6 +346,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/pthread_rwlockattr_init.c \
         thread/pthread_rwlockattr_setpshared.c \
         thread/pthread_setcancelstate.c \
+        thread/pthread_setcanceltype.c \
         thread/pthread_setspecific.c \
         thread/pthread_self.c \
         thread/pthread_spin_destroy.c \

expected/wasm32-wasip1-threads/defined-symbols.txt

@@ -1002,6 +1002,7 @@ pthread_barrierattr_destroy
 pthread_barrierattr_getpshared
 pthread_barrierattr_init
 pthread_barrierattr_setpshared
+pthread_cancel
 pthread_cond_broadcast
 pthread_cond_destroy
 pthread_cond_init
@@ -1056,6 +1057,7 @@ pthread_rwlockattr_init
 pthread_rwlockattr_setpshared
 pthread_self
 pthread_setcancelstate
+pthread_setcanceltype
 pthread_setspecific
 pthread_spin_destroy
 pthread_spin_init

libc-top-half/musl/src/thread/pthread_cancel.c

@@ -3,6 +3,7 @@
 #include "pthread_impl.h"
 #include "syscall.h"
 
+#ifdef __wasilibc_unmodified_upstream
 hidden long __cancel(), __syscall_cp_asm(), __syscall_cp_c();
 
 long __cancel()
@@ -99,3 +100,9 @@ int pthread_cancel(pthread_t t)
 	}
 	return pthread_kill(t, SIGCANCEL);
 }
+#else
+int pthread_cancel(pthread_t t)
+{
+	return ENOTSUP;
+}
+#endif

libc-top-half/musl/src/thread/pthread_setcancelstate.c

@@ -2,12 +2,10 @@
 
 int __pthread_setcancelstate(int new, int *old)
 {
-#if defined(__wasilibc_unmodified_upstream) || defined(_REENTRANT)
 	if (new > 2U) return EINVAL;
 	struct pthread *self = __pthread_self();
 	if (old) *old = self->canceldisable;
 	self->canceldisable = new;
-#endif
 	return 0;
 }