Change the cmake variable COMPILER_RT_CAN_EXECUTE_TESTS to be an option so that it can overwritten.

The reason why this simple change is needed is that I am trying to set up a
quick cmake/ninja based buildbot and apple-clang does not support using the
sanitizers currently.

The default behavior follows exactly what was there before implying that no ones
builds should be affected at all.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@178455 91177308-0d34-0410-b5e6-96231b3b80d8