Add support for the `mandatory` param to `find_cc_toolchain` in `@rules_cc`

PiperOrigin-RevId: 671758214
Change-Id: I37d0ca1f9b083513cb5933984153e1ba608313ca

cc/find_cc_toolchain.bzl

@@ -55,15 +55,18 @@ Bazel version is not needed), it's enough to only keep the toolchain type.
 
 CC_TOOLCHAIN_TYPE = "@bazel_tools//tools/cpp:toolchain_type"  # copybara-use-repo-external-label
 
-def find_cc_toolchain(ctx):
+def find_cc_toolchain(ctx, *, mandatory = True):
     """
 Returns the current `CcToolchainInfo`.
 
     Args:
       ctx: The rule context for which to find a toolchain.
+      mandatory: (bool) If this is set to False, this function will return None
+        rather than fail if no toolchain is found.
 
     Returns:
-      A CcToolchainInfo.
+      A CcToolchainInfo or None if the c++ toolchain is declared as
+      optional, mandatory is False and no toolchain has been found.
     """
 
     # Check the incompatible flag for toolchain resolution.
@@ -72,6 +75,9 @@ Returns the current `CcToolchainInfo`.
             fail("In order to use find_cc_toolchain, your rule has to depend on C++ toolchain. See find_cc_toolchain.bzl docs for details.")
         toolchain_info = ctx.toolchains[CC_TOOLCHAIN_TYPE]
         if toolchain_info == None:
+            if not mandatory:
+                return None
+
             # No cpp toolchain was found, so report an error.
             fail("Unable to find a CC toolchain using toolchain resolution. Target: %s, Platform: %s, Exec platform: %s" %
                  (ctx.label, ctx.fragments.platform.platform, ctx.fragments.platform.host_platform))
@@ -84,6 +90,8 @@ Returns the current `CcToolchainInfo`.
         return ctx.attr._cc_toolchain[cc_common.CcToolchainInfo]
 
     # We didn't find anything.
+    if not mandatory:
+        return None
     fail("In order to use find_cc_toolchain, your rule has to depend on C++ toolchain. See find_cc_toolchain.bzl docs for details.")
 
 def find_cpp_toolchain(ctx):