signal-desktop.webrtc: fix cross compilation

all the new flags here were copied from nixpkgs' chromium package,
at pkgs/applications/networking/browsers/chromium/common.nix.

pkgs/by-name/si/signal-desktop/webrtc.nix

@@ -1,6 +1,7 @@
 {
   stdenv,
   lib,
+  buildPackages,
   ninja,
   gn,
   python3,
@@ -13,6 +14,24 @@
   rustc,
 }:
 
+let
+  chromiumRosettaStone = {
+    cpu =
+      platform:
+      let
+        name = platform.parsed.cpu.name;
+      in
+      (
+        {
+          "x86_64" = "x64";
+          "i686" = "x86";
+          "arm" = "arm";
+          "aarch64" = "arm64";
+        }
+        .${name} or (throw "no chromium Rosetta Stone entry for cpu: ${name}")
+      );
+  };
+in
 stdenv.mkDerivation (finalAttrs: {
   pname = "signal-webrtc";
   version = finalAttrs.gclientDeps."src".path.rev;
@@ -59,7 +78,11 @@ stdenv.mkDerivation (finalAttrs: {
   '';
 
   gnFlags = [
+    # webrtc uses chromium's `src/build/BUILDCONFIG.gn`. many of these flags
+    # are copied from pkgs/applications/networking/browsers/chromium/common.nix.
     ''target_os="linux"''
+    ''target_cpu="${chromiumRosettaStone.cpu stdenv.hostPlatform}"''
+    ''pkg_config="${stdenv.cc.targetPrefix}pkg-config"''
     "use_sysroot=false"
     "is_clang=false"
     "treat_warnings_as_errors=false"
@@ -78,7 +101,20 @@ stdenv.mkDerivation (finalAttrs: {
     "rtc_include_tests=false"
     "rtc_enable_protobuf=false"
 
-    ''rust_sysroot_absolute="${rustc}"''
+    ''rust_sysroot_absolute="${buildPackages.rustc}"''
+
+    # Build using the system toolchain (for Linux distributions):
+    #
+    # What you would expect to be called "target_toolchain" is
+    # actually called either "default_toolchain" or "custom_toolchain",
+    # depending on which part of the codebase you are in; see:
+    # https://chromium.googlesource.com/chromium/src/build/+/3c4595444cc6d514600414e468db432e0f05b40f/config/BUILDCONFIG.gn#17
+    ''custom_toolchain="//build/toolchain/linux/unbundle:default"''
+    ''host_toolchain="//build/toolchain/linux/unbundle:default"''
+  ]
+  ++ lib.optionals (stdenv.buildPlatform != stdenv.hostPlatform) [
+    ''host_toolchain="//build/toolchain/linux/unbundle:host"''
+    ''v8_snapshot_toolchain="//build/toolchain/linux/unbundle:host"''
   ];
   ninjaFlags = [ "webrtc" ];