build-script: flush compiler-rt embedded builtins logs in llvm.py (swiftlang#82872)

Current use of `print` without `flush=True` outputs these logs at the end of a `build-script` run, after the supposedly final `--- Build Script Analyzer ---` output, which is quite confusing when reading logs.

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/products/llvm.py

@@ -141,7 +141,7 @@ def copy_embedded_compiler_rt_builtins_from_darwin_host_toolchain(
                                                      'lib', 'darwin')
                 print('copying compiler-rt embedded builtins from {}'
                       ' into the local clang build directory {}.'.format(
-                          host_cxx_builtins_dir, dest_builtins_dir))
+                          host_cxx_builtins_dir, dest_builtins_dir), flush=True)
 
                 for _os in ['ios', 'watchos', 'tvos', 'xros']:
                     # Copy over the device .a when necessary