[libcxx] Codesign test executables if necessary

If LLVM_CODESIGNING_IDENTITY is set, test executables need to be
codesigned.

Differential Revision: https://reviews.llvm.org/D66496

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@371126 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed