build: derive NODE_ARCH from target_cpu in the GN build

unofficial.gni special-cased ia32 on current_cpu while every other
architecture used target_cpu. When node is also compiled in a host
toolchain of a cross build (for example node_mksnapshot running in
V8's snapshot toolchain, which is x86-hosted for 32-bit arm targets),
current_cpu is the host's, so NODE_ARCH became "ia32" for an
arm-targeted build and process.arch deserialized as 'ia32' on the
device. Key the special case off target_cpu like the rest.

Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/65491
Refs: https://github.com/electron/electron/pull/52959
Reviewed-By: Filip Skokan <panva.ip@gmail.com>

unofficial.gni

@@ -105,7 +105,7 @@ template("node_gn_build") {
       "-Wno-unused-function",
     ]
 
-    if (current_cpu == "x86") {
+    if (target_cpu == "x86") {
       node_arch = "ia32"
     } else {
       node_arch = target_cpu