[LinkerScript] parse OUTPUT_ARCH.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@202100 91177308-0d34-0410-b5e6-96231b3b80d8
[test] use llvm-config to drive testing in assert mode/debug mode

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@202101 91177308-0d34-0410-b5e6-96231b3b80d8