[ptr-compr][arm64] Fix to timeout of regress-708247

Skipping rather than reverting the CL that introduced the slowness, due to
the fact that said CL is a Stable blocker fix.

Cq-Include-Trybots: luci.v8.try:v8_linux64_arm64_pointer_compression_rel_ng
Bug: v8:9256
Change-Id: I8f8b57c415d2b54fe57c43a87e42990909295c57
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1615260Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61603}

test/mjsunit/mjsunit.status

@@ -470,6 +470,9 @@
 
   # BUG(v8:7247).
   'regress/regress-779407': [PASS, SLOW, NO_VARIANTS],
+
+  # BUG(v8:9256). Slow with pointer compression.
+  'regress/regress-708247': [PASS, ['pointer_compression', SLOW]],
 }],  # 'arch == arm64'
 
 ['arch == arm64 and mode == debug and simulator_run', {