The important part (for the Smi::kZero compile failure at least) was v8_use_snapshot = false.

Hmm - but the linux bot that broke has snapshot on:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20shared/builds/13751/steps/generate_build_files/logs/stdout

The windows bot in the CQ testing shared libraries has snapshot off and didn't fail.

To clarify: I could only reproduce the failure locally when building with snapshots off.

I can repro with snapshot on. Just like the bot built. Sequence of steps:

git co 7db0ecdec3cf330766575cb7973b983f3f1e3020
gclient sync
tools/dev/v8gen.py -m client.v8 -b "V8 Linux - shared" shared
ninja -C out.gn/shared -j1000

[1296/1393] LINK ./d8
FAILED: d8
[...]

$ cat out.gn/shared/args.gn
is_component_build = true
is_debug = false
target_cpu = "x86"
use_goma = true
v8_enable_verify_heap = true
v8_test_isolation_mode = "prepare"

# Additional goma args:
use_goma=true