[CMake] Quote variables where "TARGET" may be a value

In CMake, "TARGET" is a special keyword. But it's also an LLVM component, which
means downstreams may request "target" or "TARGET" from CMake. Quote such input
so "TARGET" is interpreted as a string rather than a keyword.

This is a followup to 75a0502f (D150884).

Fixes Meson's test suite and an issue which manifested identically to #61436
but appears to have been a slightly different problem.

Bug: https://github.com/mesonbuild/meson/issues/11642
Bug: https://github.com/llvm/llvm-project/issues/61436

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D152121