feat(start): _start, __libc_start_main, exit/atexit/environ

crt/x86_64/crt1.s

@@ -0,0 +1,34 @@
+/*
+ * vlibc — x86_64 _start (todo 3).
+ *
+ * The kernel enters the program here with the initial stack holding, in
+ * order from %rsp: argc, argv[0..argc-1], a NULL argv terminator, the
+ * environment strings, a NULL envp terminator, and the auxiliary vector.
+ * The kernel leaves %rsp 16-byte aligned at entry.
+ *
+ * _start forwards (main, argc, argv, envp) to __libc_start_main, matching
+ * vlibc's startup signature (src/start/start.h). The frame pointer is
+ * zeroed first so the outermost frame terminates backtraces, and %rsp is
+ * 16-byte aligned at the call site per the SysV AMD64 ABI. __libc_start_main
+ * never returns (it ends in exit), so the hlt loop below is unreachable; it
+ * exists only to satisfy the assembler and to fault loudly (hlt in user
+ * mode raises #GP) if a broken build ever falls through.
+ */
+
+    .text
+    .global _start
+    .type _start, @function
+_start:
+    xor %ebp, %ebp              # outermost frame: zero frame pointer
+    mov %rsp, %r8               # keep the raw initial stack for envp
+    pop %rsi                    # rsi = argc
+    mov %rsp, %rdx              # rdx = argv
+    lea 16(%r8, %rsi, 8), %rcx  # rcx = envp = initial rsp + 8 + 8*(argc+1)
+    lea main(%rip), %rdi        # rdi = &main
+    and $-16, %rsp              # ABI: rsp % 16 == 0 at the call site
+    call __libc_start_main      # _Noreturn: routes main's result into exit
+1:  hlt
+    jmp 1b
+    .size _start, . - _start
+
+    .section .note.GNU-stack, "", @progbits