[build-script] When building compiler-rt, follow LLVM's lead by specifying that we are using LLVM's external project mechanism.

This matches the LLVM build more closely preventing potential build issues
coming up.

rdar://26284108

Author: gottesmm

utils/build-script-impl

@@ -524,6 +524,7 @@ function set_deployment_target_based_options() {
                 -DLLVM_HOST_TRIPLE:STRING="${llvm_host_triple}"
                 -DLLVM_ENABLE_LIBCXX:BOOL=TRUE
                 -DLLVM_TOOL_COMPILER_RT_BUILD:BOOL="$(false_true ${SKIP_COMPILER_RT})"
+                -DLLVM_BUILD_EXTERNAL_COMPILER_RT:BOOL="$(false_true ${SKIP_COMPILER_RT})"
                 -DCOMPILER_RT_ENABLE_IOS:BOOL=FALSE
                 -DCOMPILER_RT_ENABLE_WATCHOS:BOOL=FALSE
                 -DCOMPILER_RT_ENABLE_TVOS:BOOL=FALSE