Add --disable_legacy_crosstool_fields to host configuration

https://github.com/bazelbuild/bazel/issues/6861
https://github.com/bazelbuild/bazel/issues/5883

RELNOTES: None
PiperOrigin-RevId: 229770248

tools/cpp/lib_cc_configure.bzl

diff --git a/tools/cpp/lib_cc_configure.bzl b/tools/cpp/lib_cc_configure.bzl
index ebb0a05..3bce481 100644
--- a/tools/cpp/lib_cc_configure.bzl
+++ b/tools/cpp/lib_cc_configure.bzl
@@ -198,3 +198,7 @@
     """Returns True if CC_CONFIGURE_DEBUG is set to 1."""
     env = repository_ctx.os.environ
     return "CC_CONFIGURE_DEBUG" in env and env["CC_CONFIGURE_DEBUG"] == "1"
+
+def build_flags(flags):
+    """Convert `flags` to a string of flag fields."""
+    return "\n".join(["        flag: '" + flag + "'" for flag in flags])