[compiler-rt] Switch from llvm-config to find_package(LLVM)

Replace the use of the deprecated `llvm-config` tool with LLVM's CMake
files for detecting LLVM in standalone builds.

Differential Revision: https://reviews.llvm.org/D137024

GitOrigin-RevId: 23c665371a98c7a16765d12bf2db19b0626b7181
2 files changed