[CodeGen] Implicitly set stackrealign on the main function, if custom stack alignment is used

If using a custom stack alignment, one is expected to make sure
that all callers provide such alignment, or realign the stack in
all entry points (and callbacks).

Despite this, the compiler can assume that the main function will
need realignment in these cases, since the startup routines calling
the main function most probably won't provide the custom alignment.

This matches what GCC does in similar cases; if compiling with
-mincoming-stack-boundary=X -mpreferred-stack-boundary=X, GCC normally
assumes such alignment on entry to a function, but specifically for
the main function still does realignment.

Differential Revision: https://reviews.llvm.org/D51026

llvm-svn: 340334