Use function attribute "stackrealign" to decide whether stack

Use function attribute "stackrealign" to decide whether stack
realignment should be forced.

With this commit, we can now force stack realignment when doing LTO and
do so on a per-function basis. Also, add a new cl::opt option
"stackrealign" to CommandFlags.h which is used to force stack
realignment via llc's command line.

Out-of-tree projects currently using -force-align-stack to force stack
realignment should make changes to attach the attribute to the functions
in the IR.

Differential Revision: http://reviews.llvm.org/D11814

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247450 91177308-0d34-0410-b5e6-96231b3b80d8