[sdk] append target cpu to the gen_snapshot binaries

gen_snapshot is target cpu specific and must not be overwritten in the
SDK merge phase.

Bug: DX-1151
Change-Id: I4b5b030e85bb7c02f753e29b940e44b751debb45
diff --git a/runtime/dart/BUILD.gn b/runtime/dart/BUILD.gn
index dbd3676..eb53739 100644
--- a/runtime/dart/BUILD.gn
+++ b/runtime/dart/BUILD.gn
@@ -77,11 +77,11 @@
     },
     {
       source = prebuilt_gen_snapshot
-      dest = "tools/dart_prebuilts/gen_snapshot.snapshot"
+      dest = "tools/dart_prebuilts/gen_snapshot.${current_cpu}"
     },
     {
       source = prebuilt_gen_snapshot_product
-      dest = "tools/dart_prebuilts/gen_snapshot_product.snapshot"
+      dest = "tools/dart_prebuilts/gen_snapshot_product.${current_cpu}"
     },
     {
       source = "$root_out_dir/dart_runner_patched_sdk/platform_strong.dill"