driver: build C/C++ with -ffunction-sections -fdata-sections when split sections is enabled

When -fsplit-sections is passed to GHC, pass -ffunction-sections
-fdata-sections to gcc/clang when building C/C++. Previously,
-fsplit-sections was only respected by the NCG/LLVM backends, but not
the unregisterised backend; the GHC driver did not pass
-fdata-sections and -ffunction-sections to the C compiler, which
resulted in excessive executable sizes.

Fixes #23381.

-------------------------
Metric Decrease:
    size_hello_artifact
    size_hello_unicode
-------------------------

compiler/GHC/Driver/Pipeline/Execute.hs

@@ -487,6 +487,13 @@ runCcPhase cc_phase pipe_env hsc_env location input_fn = do
                     , not $ target32Bit (targetPlatform dflags)
                     ]
 
+                 -- if -fsplit-sections is enabled, we should also
+                 -- build with these flags.
+                 ++ (if gopt Opt_SplitSections dflags &&
+                      platformOS (targetPlatform dflags) /= OSDarwin
+                        then ["-ffunction-sections", "-fdata-sections"]
+                        else [])
+
           -- Stub files generated for foreign exports references the runIO_closure
           -- and runNonIO_closure symbols, which are defined in the base package.
           -- These symbols are imported into the stub.c file via RtsAPI.h, and the