Allow users to choose identity used to sign tools.

Summary:
No change if the identity isn't defined by the makefile.

Reviewers: echristo

Differential Revision: http://llvm-reviews.chandlerc.com/D632

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180240 91177308-0d34-0410-b5e6-96231b3b80d8