crtbegin: Correctly align ESP to 16 for __i386__

The (lowest) address of the argument area (aka ESP immediately prior to
the call instruction) must be aligned to 0 mod 16. Here, it is aligned to
12 mod 16.

From the SysV ABI doc (2.2.2 The Stack Frame)

"""The end of the input argument area shall be aligned on a 16 (32, if
__m256 is passed on stack) byte boundary. In other words, the value
(%esp + 4) is always a multiple of 16 (32) when control is transferred to
the function entry point."""

Test: extract code into a separate C file and verify stack alignment in a
  "start_main" function
Test: use the upcoming NDK r17-beta1 (with new Bionic crtbegin*.o files)
  with an M-23 x86 system image, check alignment in main (compiled with
  Clang not GCC, compiled w/o -mstackrealign)
Bug: b/73140672

Change-Id: Ia8d93fe5668d0a514a9fd22c40bf8362805111e6