Support external toolchain on Windows for aot compiler (#3911)

allowing custom ARC toolchain on Windows

build-scripts/build_llvm.py

@@ -102,12 +102,27 @@ def build_llvm(llvm_dir, platform, backends, projects, use_clang=False, extra_fl
         "default": [],
     }
 
+    experimental_backends = ["ARC", "Xtensa"]
+    normal_backends = [s for s in backends if s not in experimental_backends]
+
     LLVM_TARGETS_TO_BUILD = [
-        '-DLLVM_TARGETS_TO_BUILD:STRING="' + ";".join(backends) + '"'
-        if backends
+        '-DLLVM_TARGETS_TO_BUILD:STRING="' + ";".join(normal_backends) + '"'
+        if normal_backends
         else '-DLLVM_TARGETS_TO_BUILD:STRING="AArch64;ARM;Mips;RISCV;X86"'
     ]
 
+    # if not on ARC platform, but want to add expeirmental backend ARC as target
+    if platform != "ARC" and "ARC" in backends: 
+        LLVM_TARGETS_TO_BUILD.extend(
+            LLVM_EXTRA_COMPILE_OPTIONS["arc"]
+        )
+
+    if platform != "Xtensa" and "Xtensa" in backends:
+        print(
+            "Currently it's not supported to build Xtensa backend on non-Xtensa platform"
+        )
+        return None
+
     LLVM_PROJECTS_TO_BUILD = [
         '-DLLVM_ENABLE_PROJECTS:STRING="' + ";".join(projects) + '"' if projects else ""
     ]
@@ -240,6 +255,7 @@ def main():
             "X86",
             "Xtensa",
         ],
+        default=[],
         help="identify LLVM supported backends, separate by space, like '--arch ARM Mips X86'",
     )
     parser.add_argument(