error handling

src/codegen_x86_64.rs

@@ -110,6 +110,8 @@ section .bss
     _heap_head: resq 1
     _heap_tail: resq 1
     _environ: resq 1
+    _err_code: resq 1
+    _err_msg: resq 1
 
 section .text._builtin_heap_head
 _builtin_heap_head:
@@ -121,6 +123,16 @@ _builtin_heap_tail:
     lea     rax, [rel _heap_tail]
     ret
 
+section .text._builtin_err_code
+_builtin_err_code:
+    lea     rax, [rel _err_code]
+    ret
+
+section .text._builtin_err_msg
+_builtin_err_msg:
+    lea     rax, [rel _err_msg]
+    ret
+
 section .text._builtin_read64
 _builtin_read64:
     mov rax, qword [rdi]