Implement stack base initialization (#25)

Author: Blackhex

winsup/cygwin/dcrt0.cc

@@ -1020,7 +1020,6 @@ _dll_crt0 ()
      anyway, we now always move the main thread stack to the stack area
      reserved for pthread stacks.  This allows a reproducible stack space
      under our own control and avoids collision with the OS. */
-#if !defined(__aarch64__)
   if (!dynamically_loaded)
     {
       if (__in_forkee != FORKING)
@@ -1031,14 +1030,20 @@ _dll_crt0 ()
 	  PVOID stackaddr = create_new_main_thread_stack (allocationbase);
 	  if (stackaddr)
 	    {
-#if defined(__x86_64__)
 	      /* Set stack pointer to new address.  Set frame pointer to
 	         stack pointer and subtract 32 bytes for shadow space. */
+#if defined(__x86_64__)
 	      __asm__ ("\n\
 		       movq %[ADDR], %%rsp \n\
 		       movq  %%rsp, %%rbp  \n\
 		       subq  $32,%%rsp     \n"
 		       : : [ADDR] "r" (stackaddr));
+#elif defined(__aarch64__)
+	      __asm__ ("\n\
+		       mov fp, %[ADDR] \n\
+		       sub sp, fp, #32 \n"
+		       : : [ADDR] "r" (stackaddr)
+		       : "memory");
 #else
 #error unimplemented for this target
 #endif
@@ -1051,7 +1056,6 @@ _dll_crt0 ()
       else
 	fork_info->alloc_stack ();
     }
-#endif
 
   fesetenv (FE_DFL_ENV);
   _main_tls = &_my_tls;