Add arch_stk_setup function for setting up a new stack during thread creation

Some architectures, like PowerPC, need the stack to be adjusted before thread creation.
This is unnecessary on Dreamcast, so the implementation added to Dreamcast is empty.

Author: darcagn

kernel/arch/dreamcast/include/arch/stack.h

@@ -100,6 +100,15 @@ static inline uintptr_t arch_fptr_next(uintptr_t fptr) {
     return arch_fptr_ret_addr(fptr + 4);
 }
 
+/** \brief  Set up new stack before running.
+
+    This function does nothing as it is unnecessary on Dreamcast.
+
+    \param nt               A pointer to the new thread for which a stack
+                            is to be set up.
+*/
+void arch_stk_setup(kthread_t *nt);
+
 /** \brief  Do a stack trace from the current function.
 
     This function does a stack trace from the current function, printing the

kernel/arch/dreamcast/kernel/stack.c

@@ -20,6 +20,11 @@ static uintptr_t arch_stack_32m_dft = 0x8e000000;
 extern uintptr_t arch_stack_16m __attribute__((weak,alias("arch_stack_16m_dft")));
 extern uintptr_t arch_stack_32m __attribute__((weak,alias("arch_stack_32m_dft")));
 
+/* This function is unnecessary and does nothing on Dreamcast */
+void arch_stk_setup(kthread_t *nt) {
+    (void)nt;
+}
+
 /* Do a stack trace from the current function; leave off the first n frames
    (i.e., in assert()). */
 __noinline void arch_stk_trace(int n) {

kernel/thread/thread.c

@@ -362,8 +362,11 @@ int thd_remove_from_runnable(kthread_t *thd) {
 }
 
 /* New thread function; given a routine address, it will create a
-   new kernel thread with the given attributes. When the routine
-   returns, the thread will exit. Returns the new thread struct. */
+   new thread with the given attributes. When the routine returns,
+   the thread will exit. Returns the new thread struct.
+   Note that this function is also used in thd_init() to add the
+   already running kernel thread to the thread scheduler. This is
+   the only circumstance in which routine should be NULL. */
 kthread_t *thd_create_ex(const kthread_attr_t *restrict attr,
                          void *(*routine)(void *param), void *param) {
     kthread_t *nt = NULL;
@@ -431,6 +434,13 @@ kthread_t *thd_create_ex(const kthread_attr_t *restrict attr,
                                ((uint32_t)nt->stack) + nt->stack_size,
                                (uint32_t)thd_birth, params, 0);
 
+            /* Some architectures require setting up a new stack before use.
+               We won't do this if routine is NULL, however, as this means
+               we are creating the kernel thread, which is already running. */
+            if(routine) {
+                arch_stk_setup(nt);
+            }
+
             /* Create static TLS data if the thread hasn't disabled it. */
             if(real_attr.disable_tls) {
                 nt->flags |= THD_DISABLE_TLS;