Refactor tvos config, cpp20 config

Change-Id: I898f97286bc15229b8c8ce0f9670614ff0d4d274

starboard/build/config/BUILD.gn

@@ -252,4 +252,25 @@ config("default_cpp_standard") {
   if (is_host_win && !sb_is_modular) {
     cflags = [ "/std:c++17" ]
   }
+
+  if (is_apple) {
+    cflags_cc = [ "-std=gnu++17" ]
+    cflags_objcc = [ "-std=gnu++17" ]
+  }
+}
+
+config("cpp20_supported_config") {
+  if ((sb_is_modular && current_toolchain == cobalt_toolchain) || is_android) {
+    cflags_cc = [ "-std=c++20" ]
+  }
+
+  # These flags apply to non-modular windows platforms which includes win32, xb1.
+  if (is_host_win && !sb_is_modular) {
+    cflags = [ "/std:c++20" ]
+  }
+
+  if (is_apple) {
+    cflags_cc = [ "-std=gnu++20" ]
+    cflags_objcc = [ "-std=gnu++20" ]
+  }
 }

starboard/build/config/mac/BUILD.gn

@@ -43,8 +43,6 @@ config("host") {
 
 config("common") {
   arflags = [ "-no_warning_for_no_symbols" ]
-  cflags_cc = [ "-std=gnu++17" ]
-  cflags_objcc = [ "-std=gnu++17" ]
   cflags = [ "-fno-common" ]
   asmflags = [ "-fno-common" ]
   ldflags = [ "-fno-common" ]

starboard/nplb/compiler_compliance/BUILD.gn

@@ -54,15 +54,6 @@ if (sb_enable_cpp20_audit) {
   # We do this to ensure the -std=c++20 flag is added after any other -std flag
   # so it overrides any other one.
   group("cpp20_supported_config_shim") {
-    public_configs = [ ":cpp20_supported_config" ]
-  }
-
-  config("cpp20_supported_config") {
-    # These flags apply to non-modular windows platforms which includes win32, xb1.
-    if (is_host_win && !sb_is_modular) {
-      cflags_cc = [ "/std:c++20" ]
-    } else {
-      cflags_cc = [ "-std=c++20" ]
-    }
+    public_configs = [ "//starboard/build/config:cpp20_supported_config" ]
   }
 }