fix: use a dedicated section for stack

Author: kkebo

Sources/AsmSupport/aarch64/boot.S

@@ -61,9 +61,8 @@ from_el3:
     eret
 
 el1_entry:
-    // set top of stack just before our code
-    ldr x1, =_start
-    mov sp, x1
+    ldr x0, =__stack_top
+    mov sp, x0
 
 jump_to_main:
     // jump to Swift code, should not return

linker.ld

@@ -11,7 +11,13 @@ SECTIONS {
         . = ALIGN(8);
         __bss_end = .;
     }
-    __kernel_end = .;
+    __kernel_end = ALIGN(16);
+    .stack (NOLOAD) : ALIGN(16) {
+        __stack_bottom = .;
+        . += 0x20000;  /* 128 KiB */
+        __stack_top = .;
+    }
+    __kernel_image_end = .;
     .nonalloc (NOLOAD) : {
         *(.symtab)
         *(.strtab)