fix: gcc builds broken by clang-only warning flag

Closes https://github.com/dart-lang/sdk/pull/60422

GitOrigin-RevId: 8e32b0bea95193aa5ab6f61e4e3cb8224b9a7c31
Change-Id: I034d5db35873c065fa83d1407ce7674476884abb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418662
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/BUILD.gn

@@ -243,7 +243,6 @@ config("dart_config") {
     }
   } else {
     cflags = [
-      "-Wnewline-eof",  # Flutter compiles with this (to match Fuchsia).
       "-Wno-unused-parameter",
       "-Wno-unused-private-field",
       "-Wnon-virtual-dtor",
@@ -257,6 +256,7 @@ config("dart_config") {
     ]
     if (is_clang) {
       cflags += [
+        "-Wnewline-eof",  # Flutter compiles with this (to match Fuchsia).
         "-Wimplicit-fallthrough",
         "-fno-strict-vtable-pointers",  # Handle assignment updates vtable
                                         # pointers.