Register DUMPBIN on Windows when using clang-cl

[stevenlr]

This is an updated version of #363.

Without this change, it is not possible to build with clang-cl on Windows.

This is the error this PR fixes:

ERROR: C:/users/.../g2xpdmbf/external/rules_cc+/cc/private/toolchain/windows_cc_configure.bzl:756:64: An error occurred during the fetch of repository 'rules_cc++cc_configure_extension+local_config_cc':
   Traceback (most recent call last):
        File "C:/users/.../g2xpdmbf/external/rules_cc+/cc/private/toolchain/cc_configure.bzl", line 94, column 36, in cc_autoconf_impl
                configure_windows_toolchain(repository_ctx)
        File "C:/users/.../g2xpdmbf/external/rules_cc+/cc/private/toolchain/windows_cc_configure.bzl", line 922, column 35, in configure_windows_toolchain
                msvc_vars_x64 = _get_msvc_vars(repository_ctx, paths, "x64")
        File "C:/users/.../g2xpdmbf/external/rules_cc+/cc/private/toolchain/windows_cc_configure.bzl", line 756, column 64, in _get_msvc_vars
                "%{msvc_dumpbin_path_" + target_arch + "}": build_tools["DUMPBIN"],
Error: key "DUMPBIN" not found in dictionary
ERROR: no such package '@@rules_cc++cc_configure_extension+local_config_cc//': key "DUMPBIN" not found in dictionary