[NFC] Add SUPPORT_PLUGINS to add_llvm_executable()

[NFC] Add SUPPORT_PLUGINS to add_llvm_executable()

Summary:
this allows us to move logic about when it is appropriate set
LLVM_NO_DEAD_STRIP out of each tool and into add_llvm_executable,
which will enable future platform specific handling.

This is a follow on to the reverted D69356

Reviewers: hubert.reinterpretcast, beanz, lhames

Reviewed By: beanz

Subscribers: mgorny, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D69638