Set thread information earlier in exec_env creation (#3967)

For boundary checking, WAMR calls `pthread_attr_np`, which is
unfortunately quite slow on Linux when not called on the main thread
(see #3966
for discussion).

This change moves the cost of stack bounds checking earlier in the
wasm_exec_env creation process. The idea is that it's perhaps better to
pay the price when creating the execution environment rather than in the
first function call.

The original code is left in place inside
`call_wasm_with_hw_bound_check` in case the `wasm_exec_env` is created
via `wasm_runtime_spawn_exec_env`.

Author: sjamesr

core/shared/platform/common/posix/posix_thread.c

@@ -712,6 +712,13 @@ os_thread_signal_init(os_signal_handler handler)
 #if WASM_DISABLE_STACK_HW_BOUND_CHECK == 0
     sigalt_stack_base_addr = map_addr;
 #endif
+
+#if defined(os_thread_local_attribute)
+    // calculate and cache the new stack boundary.
+    // see https://github.com/bytecodealliance/wasm-micro-runtime/issues/3966
+    (void)os_thread_get_stack_boundary();
+#endif
+
     signal_handler = handler;
     thread_signal_inited = true;
     return 0;