Switch x86 begin.c to asm; align ESP correctly

Every other architecture already uses an assembly file here.

The previous code aligned ESP incorrectly, but it doesn't really matter
because everything is built with Clang's -mstackrealign, which realigns
ESP in every function prologue.

Bug: http://b/73140672#comment4
Test: lunch aosp_x86-eng; m; emulator; device boots
Test: manual
Change-Id: I921fd7848cdc611b4f8f13d1176d1983ffea952d

linker/Android.bp

@@ -104,9 +104,7 @@ cc_binary {
             version_script: "linker.generic.map",
         },
         x86: {
-            srcs: ["arch/x86/begin.c"],
-
-            cflags: ["-D__work_around_b_24465209__"],
+            srcs: ["arch/x86/begin.S"],
             version_script: "linker.generic.map",
         },
         x86_64: {

linker/arch/x86/begin.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2012 The Android Open Source Project
+ * Copyright (C) 2018 The Android Open Source Project
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -26,28 +26,18 @@
  * SUCH DAMAGE.
  */
 
-#include <stdint.h>
-#include <sys/cdefs.h>
+#include <private/bionic_asm.h>
 
-extern unsigned __linker_init(void* raw_args);
-
-__LIBC_HIDDEN__ void _start() {
+ENTRY(_start)
   // Force unwinds to end in this function.
-  asm volatile(".cfi_undefined \%eip");
+  .cfi_undefined %eip
 
-  void (*start)(void);
-
-  void* raw_args = (void*) ((uintptr_t) __builtin_frame_address(0) + sizeof(void*));
-  start = (void(*)(void))__linker_init(raw_args);
+  movl %esp, %eax         // %esp is aligned to 16 here.
+  subl $12, %esp
+  pushl %eax
+  call __linker_init      // %esp is aligned to 16 before the call.
+  addl $16, %esp
 
   /* linker init returns (%eax) the _entry address in the main image */
-  /* entry point expects sp to point to raw_args */
-
-  __asm__ (
-     "mov %0, %%esp\n\t"
-     "jmp *%1\n\t"
-     : : "r"(raw_args), "r"(start) :
-  );
-
-  /* Unreachable */
-}
+  jmp *%eax
+END(_start)