WIP: add support for pthread_atfork

This change is an experiment to enable `pthread_atfork` support under
the mistaken assumption that it may have been called during thread
creation. I no longer believe it is, so this is not a good idea.

Here's how the registered handlers are executed, though:
+ prepare handlers are called in the parent before the fork with
  `__fork_handler(-1)`
+ parent handlers are called after the fork with `__fork_handler(0)`;
  fork returns the PID to the parent, which is negated (`!ret`)
+ child handlers are called after the fork with `__fork_handler(1)`;
  fork returns 0 to the child, which is negated (`!ret`)

Makefile

@@ -206,6 +206,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/__wait.c \
         thread/__timedwait.c \
         thread/default_attr.c \
+        thread/pthread_atfork.c \
         thread/pthread_attr_destroy.c \
         thread/pthread_attr_get.c \
         thread/pthread_attr_init.c \

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

@@ -63,6 +63,7 @@ __flbf
 __floatscan
 __fmodeflags
 __fopen_rb_ca
+__fork_handler
 __fpending
 __fpurge
 __fputwc_unlocked
@@ -977,6 +978,7 @@ program_invocation_name
 program_invocation_short_name
 pselect
 psignal
+pthread_atfork
 pthread_attr_destroy
 pthread_attr_getdetachstate
 pthread_attr_getguardsize

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

@@ -315,6 +315,7 @@ hidden void __wasi_thread_start_C(int tid, void *p)
 	// without waiting.
 	atomic_store((atomic_int *) &(self->tid), tid);
 	// Execute the user's start function.
+	__fork_handler(1);
 	__pthread_exit(args->start_func(args->start_arg));
 }
 #endif
@@ -540,7 +541,9 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	 * of the current module and start executing the entry function. The
 	 * wasi-threads specification requires the module to export a
 	 * `wasi_thread_start` function, which is invoked with `args`. */
+	__fork_handler(-1);
 	ret = __wasi_thread_spawn((void *) args);
+	__fork_handler(0);
 #endif
 
 #ifdef __wasilibc_unmodified_upstream