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