[Apple Silicon] [build-script] Build compiler-rt even when cross-compiling

clang and compiler-rt are rev-locked and should match.  Currently, we build clang (and use it to bootstrap the rest) but don't build compiler-rt.  We should.

This doesn't completely achieve that: this only makes us build compiler-rt for the local host.

This used to be pretty much impossible, but, since then, compiler-rt cmake support was rewritten.

We should build it for the other hosts as well, but that's trickier, because we'd need to make the cross-compiled compiler-rt build use a separate just-built clang (from the local host). We do that for the other projects, but compiler-rt is special (because it's cmake'd as a runtime part of clang/llvm).

Author: shahmishal

utils/build-script-impl

@@ -649,10 +649,19 @@ function set_build_options_for_host() {
     esac
 
 
-    llvm_cmake_options+=(
-        -DLLVM_TOOL_COMPILER_RT_BUILD:BOOL="$(false_true ${SKIP_BUILD_COMPILER_RT})"
-        -DLLVM_BUILD_EXTERNAL_COMPILER_RT:BOOL="$(false_true ${SKIP_BUILD_COMPILER_RT})"
-    )
+    # We don't currently support building compiler-rt for cross-compile targets.
+    # It's not clear that's useful anyway.
+    if is_cross_tools_host "${host}"; then
+      llvm_cmake_options+=(
+          -DLLVM_TOOL_COMPILER_RT_BUILD:BOOL=FALSE
+          -DLLVM_BUILD_EXTERNAL_COMPILER_RT:BOOL=FALSE
+      )
+    else
+      llvm_cmake_options+=(
+          -DLLVM_TOOL_COMPILER_RT_BUILD:BOOL="$(false_true ${SKIP_BUILD_COMPILER_RT})"
+          -DLLVM_BUILD_EXTERNAL_COMPILER_RT:BOOL="$(false_true ${SKIP_BUILD_COMPILER_RT})"
+      )
+    fi
 
     # If we are asked to not generate test targets for LLVM and or Swift,
     # disable as many LLVM tools as we can. This improves compile time when
@@ -861,11 +870,6 @@ if [[ "${CMAKE_GENERATOR}" == "Xcode" ]]; then
     SKIP_BUILD_COMPILER_RT=1
 fi
 
-# FIXME: We currently do not support cross-compiling swift with compiler-rt.
-if [[ "${CROSS_COMPILE_HOSTS}" ]]; then
-    SKIP_BUILD_COMPILER_RT=1
-fi
-
 if [[ "${SKIP_RECONFIGURE}" ]]; then
     RECONFIGURE=""
 fi