Use __wasilibc_subtask_await helper in more locations (WebAssembly#769)

Minor refactoring, avoids some conditional logic in one or two
locations.

Author: alexcrichton

libc-bottom-half/cloudlibc/src/libc/time/clock_nanosleep.c

@@ -62,9 +62,7 @@ int clock_nanosleep(clockid_t clock_id, int flags, const struct timespec *rqtp,
   if (flags & TIMER_ABSTIME) {
     monotonic_clock_wait_until(duration);
   } else {
-    wasip3_subtask_status_t status = monotonic_clock_wait_for(duration);
-    if (WASIP3_SUBTASK_STATE(status) != WASIP3_SUBTASK_RETURNED)
-      __wasilibc_subtask_block_on_and_drop(WASIP3_SUBTASK_HANDLE(status));
+    __wasilibc_subtask_await(monotonic_clock_wait_for(duration));
   }
 #endif
   return 0;

libc-bottom-half/cloudlibc/src/libc/unistd/pwrite.c

@@ -87,8 +87,7 @@ ssize_t pwrite(int fildes, const void *buf, size_t nbyte, off_t offset) {
   // Wait for the subtask to resolve now that the writer half is closed and if
   // we failed to write bytes (0 bytes written) and the result is an error we
   // can return -1.
-  if (WASIP3_SUBTASK_STATE(subtask_status) != WASIP3_SUBTASK_RETURNED)
-    __wasilibc_subtask_block_on_and_drop(WASIP3_SUBTASK_HANDLE(subtask_status));
+  __wasilibc_subtask_await(subtask_status);
   if (ret == 0 && result.is_err) {
     translate_error(result.val.err);
     return -1;