compiler-rt: use the current host architecture for the runtime targets

This will ensure all the test executables generated by `check-*` targets
can run on the current machine.

Addresses rdar://150469492

Author: edymtt

utils/swift_build_support/swift_build_support/products/llvm.py

@@ -313,7 +313,7 @@ def build(self, host_target):
             llvm_cmake_options.define(
                 'SANITIZER_COMMON_LINK_FLAGS:STRING', '-Wl,-z,undefs')
 
-        builtins_runtimes_target_for_darwin = 'arm64-apple-darwin'
+        builtins_runtimes_target_for_darwin = f'{arch}-apple-darwin'
         if system() == "Darwin":
             llvm_cmake_options.define(
                 f'BUILTINS_{builtins_runtimes_target_for_darwin}_'