[cmake] Add ability to customize (and skip) debugserver codesign

Summary:
This adds the ability to customize the debugserver codesign process via cmake cache variable. The
user can set the codesign indentity (with the default being the customary lldb_codesign), and if
the identity is set to a empty string, the codesign step is skipped completely.

We needed the last feature to enable building lldb on buildservers which do not have the right
certificates installed.

Reviewers: sas, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D20623

llvm-svn: 270832