[MC] When using bundle aligment, align sections to bundle size

Summary:
Bundle aligment requires that the functions always start at an aligned address.
Usually this is ensured by the compiler, but assembly code does not always
begin with a .align directive.

This change ensures that sections get the correct alignment if they contain
any instructions and bundling is enabled. (It also makes LLVM match the
behavior of GNU as).

Differential Revision: http://reviews.llvm.org/D9131

llvm-svn: 235365