[lldb] LLDB_TEST_ARCH -> LLDB_TEST_TRIPLE (#191204)
Change the CMake variable from LLDB_TEST_ARCH to LLDB_TEST_TRIPLE. I'm
working on adding support for cross compiling our test suite to
WebAssembly, where the triple (i.e. `wasm32-wasip1`) is of importance
while building the inferiors. Given that the triple is a superset of the
arch, it is strictly more expressive.
