[compiler-rt] Use ZLIB_FOUND not LLVM_ENABLE_ZLIB

LLVM_ENABLE_ZLIB is set to On by default and does not imply that ZLIB
was actually found, only being set to FORCE_ON would imply that it was
found before control reached compiler-rt/test/CMakeLists.txt. Instead,
use ZLIB_FOUND which guarantees that zlib is available.

Differential Revision: https://reviews.llvm.org/D151229