Add a note about using "Differential Revision:" in commit messages

I noticed this convention from the commit logs.  It seems like it
would be useful to document it, to encourage other committers to link
back to code reviews in their commits.

Differential Revision: http://llvm-reviews.chandlerc.com/D2678

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201160 91177308-0d34-0410-b5e6-96231b3b80d8
If LDPO_PIE is not defined (before binutils 2.23 version), 
use the hardcoded declaration 3
See r201110 for the initial change



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201161 91177308-0d34-0410-b5e6-96231b3b80d8
Improve the declaration when LDPO_PIE is not available.
Thanks to İsmail Dönmez for the better declaration.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201163 91177308-0d34-0410-b5e6-96231b3b80d8