Add fastbuild_compile_flags to unix_cc_toolchain_config

Copybara Import from #301

BEGIN_PUBLIC
Add fastbuild_compile_flags to unix_cc_toolchain_config (#301)

We already have dbg_compile_flags and opt_compile_flags.  This add symetry to enable pulling fastbuild flags out of .bazelrc files and into the toolchain.

Closes #301
END_PUBLIC

COPYBARA_INTEGRATE_REVIEW=#301 from AustinSchuh:fastbuild_flags 4e06396
PiperOrigin-RevId: 782686208
Change-Id: I02c8f569903ca938926485cd19bab75cc9414467

Author: AustinSchuh

cc/private/toolchain/unix_cc_toolchain_config.bzl

@@ -389,6 +389,15 @@ def _impl(ctx):
                     ),
                 ] if ctx.attr.compile_flags else []),
             ),
+            flag_set(
+                actions = all_compile_actions,
+                flag_groups = ([
+                    flag_group(
+                        flags = ctx.attr.fastbuild_compile_flags,
+                    ),
+                ] if ctx.attr.fastbuild_compile_flags else []),
+                with_features = [with_feature_set(features = ["fastbuild"])],
+            ),
             flag_set(
                 actions = all_compile_actions,
                 flag_groups = ([
@@ -1947,6 +1956,7 @@ cc_toolchain_config = rule(
         "cxx_flags": attr.string_list(),
         "dbg_compile_flags": attr.string_list(),
         "extra_flags_per_feature": attr.string_list_dict(),
+        "fastbuild_compile_flags": attr.string_list(),
         "host_system_name": attr.string(mandatory = True),
         "link_flags": attr.string_list(),
         "link_libs": attr.string_list(),