lit: Use sys.executable for executing builtin commands

lit: Use sys.executable for executing builtin commands

Summary:
The python executable may not exist on all systems so use sys.executable
instead.

Reviewers: ddunbar, stella.stamenova

Subscribers: delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D51511

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341244 91177308-0d34-0410-b5e6-96231b3b80d8