[Backtracing] Add target-arch to lit.cfg.

This must have ended up in the wrong PR in the stack.  It is needed in
this one so that we can locate libswiftCore.

rdar://103071801

Author: al45tair

test/lit.cfg

@@ -1759,6 +1759,9 @@ config.substitutions.append(('%module-target-future', target_future))
 # Add 'target-sdk-name' as the name for platform-specific directories
 config.substitutions.append(('%target-sdk-name', config.target_sdk_name))
 
+# Also add 'target-arch' so we can locate some things inside the build tree
+config.substitutions.append(('%target-arch', target_arch))
+
 # Add 'stdlib_dir' as the path to the stdlib resource directory
 stdlib_dir = os.path.join(config.swift_lib_dir, "swift")
 if run_os == 'maccatalyst':