added the option to customize _start

tests/start_override.expected

@@ -0,0 +1,2 @@
+hello world
+24

tests/start_override.sl

@@ -0,0 +1,44 @@
+import stdlib.sl
+
+:asm _start {
+    mov rdi, [rsp]
+    lea rsi, [rsp+8]
+    mov [rel sys_argc], rdi
+    mov [rel sys_argv], rsi
+    lea r12, [rel dstack_top]
+    mov r15, r12
+    lea r13, [rel rstack_top]
+    ; print "hello world\n" before calling main using runtime `print_buf`
+    mov byte [rel print_buf], 'h'
+    mov byte [rel print_buf + 1], 'e'
+    mov byte [rel print_buf + 2], 'l'
+    mov byte [rel print_buf + 3], 'l'
+    mov byte [rel print_buf + 4], 'o'
+    mov byte [rel print_buf + 5], ' '
+    mov byte [rel print_buf + 6], 'w'
+    mov byte [rel print_buf + 7], 'o'
+    mov byte [rel print_buf + 8], 'r'
+    mov byte [rel print_buf + 9], 'l'
+    mov byte [rel print_buf + 10], 'd'
+    mov byte [rel print_buf + 11], 10
+    lea rsi, [rel print_buf]
+    mov rdx, 12
+    mov rax, 1
+    mov rdi, 1
+    syscall
+    call main
+    mov rax, 0
+    cmp r12, r15
+    je .no_exit_value
+    mov rax, [r12]
+    add r12, 8
+    .no_exit_value:
+    mov rdi, rax
+    mov rax, 60
+    syscall
+};
+
+word main
+    24 puti cr
+end
+