[vm] Fix set_uprobe script after offsets_extractor was coverted to json

Also add a '--dry-run' parameter as running it as sudo doesn't work
for me (it times out when it tries to build because of the RBE stuff I
think).

Now I can run something like

```
pkg/vm/tool/precompiler2 --dwarf-stack-traces --generate-probe-points path/to/file.dart path/to/file.aot
out/ReleaseX64/dart-sdk/bin/dart runtime/tools/profiling/bin/set_uprobe.dart alloc AllocationProbePoint path/to/file.aot --dry-run | sudo tee "/sys/kernel/tracing/uprobe_events"
cp out/ReleaseX64/dartaotruntime out/ReleaseX64/dart-sdk/bin/dartaotruntimeNotProduct
sudo perf record -g -e uprobes:alloc out/ReleaseX64/dart-sdk/bin/dartaotruntimeNotProduct path/to/file.aot <args>
sudo chmod 0755 perf.data
```

Change-Id: I189c2c08ec9cef5e61698e4fb9c9444494d20815
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450040
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>