chore: update mksnapshot and ffmpeg for GN (#14738)

* chore: update mksnapshot and ffmpeg for GN

* fix typo

* add non-native builds to CI

* split mksnapshot build and store steps

* Fix config order

* Use different targets for mksnapshot and native_mksnapshot

mksnapshot gets built in electron_app as part of the v8 build, so copy it from there

* Revert "Use different targets for mksnapshot and native_mksnapshot"

This reverts commit 6a1a50aa690124626b4357751949397298aeac95.

* Try specifying v8_snapshot_toolchain

* Fix v8_snapshot_toolchain reference

* Fix artifact locations

* Test native mksnapshot

patches/common/v8/.patches.yaml

@@ -85,3 +85,7 @@ patches:
   author: Shelley Vohr <shelley.vohr@gmail.com>
   file: cherry-pick_70c4340.patch
   description: Node 10.6.0 needs it.
+-
+  author: Shelley Vohr <shelley.vohr@gmail.com>
+  file: expose_mksnapshot.patch
+  description: Needed in order to build mksnapshot on arm.

patches/common/v8/expose_mksnapshot.patch

@@ -0,0 +1,13 @@
+diff --git a/BUILD.gn b/BUILD.gn
+index 30e9ec34cf..fea543df9c 100644
+--- a/BUILD.gn
++++ b/BUILD.gn
+@@ -2904,8 +2904,6 @@ if (v8_monolithic) {
+ 
+ if (v8_use_snapshot && current_toolchain == v8_snapshot_toolchain) {
+   v8_executable("mksnapshot") {
+-    visibility = [ ":*" ]  # Only targets in this file can depend on this.
+-
+     sources = [
+       "src/snapshot/mksnapshot.cc",
+     ]