[compiler-rt] Pass CMAKE_TOOLCHAIN_FILE through to custom libc++ build

This can be used to configure runtimes builds (instead of setting flags
individually), and we need to pass it down to the custom libc++ build
for it to work correctly.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -643,6 +643,7 @@ macro(add_custom_libcxx name prefix)
     CMAKE_STRIP
     CMAKE_READELF
     CMAKE_SYSROOT
+    CMAKE_TOOLCHAIN_FILE
     LIBCXX_HAS_MUSL_LIBC
     LIBCXX_HAS_GCC_S_LIB
     LIBCXX_HAS_PTHREAD_LIB