Subject: Build: strip relative -isysroot from chromium-emitted lflags

In src/{core,pdf}/configure/BUILD.root.gn.in, lflags_remove is fed to
the rsp writer in our gn fork
(src/3rdparty/gn/src/gn/rsp_target_writer.cc, LFLAGS case) where it
runs std::regex_replace over the accumulated ldflags before writing
the rsp file. The intent, per the inline comment, is to drop sysroot
flags whose path was emitted as relative.

Such relative paths are produced by chromium's mac/linux toolchain
configs via rebase_path(sysroot, root_build_dir). For qtwebengine, the
chromium root_build_dir is build/src/core/Release/$arch, so the
rebased path is correct when read from there -- but the rsp file is
consumed by the *cmake*-driven ninja link, which runs from build/. The
two cwds differ by four levels, so the rebased path resolves to a
non-existent location, triggering -Wmissing-sysroot and a subsequent
"framework not found CoreFoundation" failure (linker falls back to a
bogus sysroot, can't find /System/Library/Frameworks under it).

The existing pattern only matched the Linux --sysroot=PATH single-
token form, so the Apple two-token form '-isysroot PATH' slipped
through untouched. Extend the alternation to cover both, and tighten
the path match to \.\./\S+ (the prior \.\./.*\S*? was effectively
\.\./.* and worked only because --sysroot= was the last token in
the Linux ldflags string -- on macOS -isysroot is followed by more
flags like -mmacos-version-min, so we need a match that stops at the
next whitespace).

Also add lflags_remove to the convert_dict target, which declares
rsp_types but had no filter; on macOS it hits the same relative
-isysroot issue as QtWebEngineCore (the linker fails on -lbsm because
the bogus sysroot wins last and ld can't resolve system libs under
it). QtWebEngineCore and QtPdf had the filter; convert_dict appears
to have been overlooked.

The good absolute -isysroot is already injected on the clang++ command
line by cmake earlier, so stripping the bad one is sufficient
(-isysroot is last-wins, and now only the good one remains).

Reproduced on aarch64-darwin with nixpkgs's apple-sdk-15.5, on
Qt 6.11.0. After this change qtwebengine builds successfully.

diff --git a/src/core/configure/BUILD.root.gn.in b/src/core/configure/BUILD.root.gn.in
index cf9a3fe..3b01162 100644
--- a/src/core/configure/BUILD.root.gn.in
+++ b/src/core/configure/BUILD.root.gn.in
@@ -107,7 +107,7 @@ if (ozone_platform_x11) {
 
 shared_library("QtWebEngineCore") {
   rsp_types = [ "objects", "archives", "libs", "ldir", "lflags" ]
-  lflags_remove = "(--sysroot=)(\\.\\./.*\\S*?)" # ignore sysroot with realative path
+  lflags_remove = "(--sysroot=|-isysroot )\\.\\./\\S+" # ignore sysroot with relative path
   configs += [
     ":cpp20_config",
     ":QtWebEngineCore_config",
@@ -846,6 +846,7 @@ if (enable_extensions) {
 if (enable_spellcheck) {
   shared_library("convert_dict") {
     rsp_types = [ "objects", "archives", "libs", "ldir", "lflags" ]
+    lflags_remove = "(--sysroot=|-isysroot )\\.\\./\\S+" # ignore sysroot with relative path
     configs += [ "//build/config/compiler:wexit_time_destructors" ]
     deps = [
       "//chrome/tools/convert_dict:lib",
diff --git a/src/pdf/configure/BUILD.root.gn.in b/src/pdf/configure/BUILD.root.gn.in
index 72d4558..c63cfda 100644
--- a/src/pdf/configure/BUILD.root.gn.in
+++ b/src/pdf/configure/BUILD.root.gn.in
@@ -51,7 +51,7 @@ config("cpp20_config") {
 static_library("QtPdf") {
   complete_static_lib = true
   rsp_types = [ "objects", "archives", "libs", "ldir", "lflags" ]
-  lflags_remove = "(--sysroot=)(\\.\\./.*\\S*?)" # ignore sysroot with realative path
+  lflags_remove = "(--sysroot=|-isysroot )\\.\\./\\S+" # ignore sysroot with relative path
   configs += [
     ":cpp20_config",
     ":QtPdf_config"