build: generate the Node startup snapshot on cross-arch builds too - #52879

node_use_node_snapshot / node_use_node_code_cache were gated to
host_cpu == target_cpu, so the targets Electron cross-builds -- linux
arm64 and win arm64 (x64 hosts) and mac x64 (arm64 hosts) -- shipped
without the embedded Node snapshot and its builtin code cache, and every
process there still ran the full Node bootstrap.

node_mksnapshot already builds in v8_snapshot_toolchain, like mksnapshot
and v8_context_snapshot_generator. Unlike those it executes target-CPU
code (the Node bootstrap runs in JS), so the requirement is a snapshot
toolchain that can run what its V8 emits: V8's arm64 simulator on x64
hosts (linux clang_x64_v8_arm64, the win host toolchain with
v8_target_cpu = arm64), or an x64 binary under Rosetta on arm64 macs,
which is what Electron CI's v8_snapshot_toolchain =
"//build/toolchain/mac:clang_x64" override for mac x64 already provides.
The one same-OS combination that cannot work is V8's default for mac
arm64 -> x64, clang_arm64_v8_x64 (arm64 binary emitting x64 code, no
simulator) -- that is the crash the old TODO in node.gni described.
Neither the snapshot nor the code cache contains machine code, so what a
cross node_mksnapshot produces is valid on the target; the js2c code
cache is already generated cross-arch the same way.

So gate both on host_os == target_os except that toolchain, and give the
node_snapshot / node_snapshot_stub source_sets the
//base:debugging_buildflags dependency that node_builtins.h's
base/dcheck_is_on.h include needs (a fresh out dir building only
third_party/electron_node:node_snapshot failed on the missing generated
header). Also brings the patch description up to date.

Verified on a linux x64 host with target_cpu = "arm64": node_mksnapshot
runs under the simulator (blob 4.9 MB, 392-entry / 2.8 MB code cache, same
shape as native), the snapshot-keyed browser js2c cache tool runs, and
electron links with the snapshot embedded. Booting it needs arm64
hardware; the js2c code-cache spec's browser assertions and
process.config.variables.node_use_node_snapshot cover it on the arm64
test legs. mac x64 and win arm64 are by construction, untested locally.

Co-authored-by: Shelley Vohr <[email protected]>

Node's GN build set NODE_ARCH to "ia32" whenever the toolchain compiling
it was x86 and to target_cpu otherwise. That is the same thing for a
normal build, but not in V8's snapshot toolchain: the arm (32-bit)
snapshot is produced by an x86-hosted node_mksnapshot, which therefore
carried NODE_ARCH="ia32", and since #52874 the arm build boots its main
process from that snapshot - so process.arch read "ia32" there while
renderers and workers said "arm". Spell the ia32 special case off
target_cpu instead, which is right in the target toolchain and in every
host toolchain that builds node for the snapshot.