Do not require deprecated create_cc_toolchain_config_info attrs

BEGIN_PUBLIC

In create_cc_toolchain_config_info, the target_system_name, target_cpu, and target_libc attributes are documented as required and deprecated. In practice, these may safely be `None`. This updates the cc_toolchain rule implementation to no longer require that these attributes are set to arbitrary values.

END_PUBLIC

PiperOrigin-RevId: 633149160
Change-Id: Ief1d3236ead9299b307ce9ace68cd295536a7b3b

cc/toolchains/impl/toolchain_config.bzl

@@ -108,9 +108,9 @@ cc_toolchain_config = rule(
         # TODO: Consider making this into a label_list that takes a
         #  cc_directory_marker rule as input.
         "cxx_builtin_include_directories": attr.string_list(),
-        "target_system_name": attr.string(mandatory = True),
-        "target_cpu": attr.string(mandatory = True),
-        "target_libc": attr.string(mandatory = True),
+        "target_system_name": attr.string(),
+        "target_cpu": attr.string(),
+        "target_libc": attr.string(),
         "compiler": attr.string(mandatory = True),
         "abi_version": attr.string(),
         "abi_libc_version": attr.string(),