libstdc++: make hardening assertions configurable from GN args

The current implementation for enabling libstdc++ hardening
assertions is done using build_overrides.gni. This is problematic
as it breaks other dependencies using //build/ as they will require
also an specific argument for that purpose.

Replace it with a GN arg, use_safe_libcxx, that uses same heuristic
(Linux without custom libc++), but can be set from command line. This
is because we cannot accurately know if libstdc++ is used, so we want
to still allow to enable the assertions.

Bug: 328287875
Change-Id: I27cf09a13af638cfc5e82bce5e3fa7391512c59c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5348001
Reviewed-by: Philipp Wollermann <philwo@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1274804}

build/config/c++/c++.gni

@@ -52,6 +52,11 @@ declare_args() {
   # true, you must also set libcxx_abi_unstable=false, which is bad for
   # performance and memory use.
   libcxx_is_shared = use_custom_libcxx && is_component_build
+
+  # In case the C++ standard library implementation used is libstdc++, then
+  # enable its own hardening checks. As we cannot determine in GN if libstdc++
+  # is used or not, by default enable it for Linux without the custom libc++.
+  use_safe_libstdcxx = is_linux && !use_custom_libcxx
 }
 
 # The saigo libc++ is distinct from the custom hermetic libc++. However, since
@@ -59,10 +64,6 @@ declare_args() {
 # enable libc++ hardening there as well.
 use_safe_libcxx = (use_custom_libcxx && enable_safe_libcxx) || is_nacl_saigo
 
-# libstdc++ has its own hardening assertions that we want to enable by default
-# in Chromium builds.
-use_safe_libstdcxx = is_linux && !use_custom_libcxx && enable_safe_libstdcxx
-
 # libc++abi needs to be exported from executables to be picked up by shared
 # libraries on certain instrumented builds.
 export_libcxxabi_from_executables =

build_overrides/build.gni

@@ -29,12 +29,6 @@ build_with_v8_embedder = build_with_chromium
 # `use_custom_libcxx = true`.
 enable_safe_libcxx = true
 
-# Enable assertions on safety checks, also in libstdc++
-#
-# In case the C++ standard library implementation used is libstdc++, then
-# enable its own hardening checks.
-enable_safe_libstdcxx = true
-
 # Features used by //base/trace_event and //services/tracing.
 declare_args() {
   # Tracing support requires //third_party/perfetto, which is not available in