Steam webviews repeatedly crash when preloading libmimalloc.so (linux)

Steam webviews repeatedly crash - showing nothing - when preloading libmimalloc.so, on linux. Although Steam works in general.

Tested with dev HEAD / 4f020e5da4552fb254ce35db1adad5c63d68d09e and v1.6.3, with and without MI_LOCAL_DYNAMIC_TLS and with and without MI_USE_CXX, fwiw.

I've also tried running with a MI_SECURE build but didn't see any notable warning spew or behavioural change.

Here's a typical dmesg - it looks like the crash is happening inside mimalloc itself:
```
CompositorTileW[10858]: segfault at 120350400060 ip 00007f99a3579d70 sp 00007f9989 es of 1 users.]
2582c0 error 4 in libmimalloc.so.1.6[7f99a3569000+1a000]
```

Start by reproducing the crash in Steam webviews with libmimalloc.so preloaded, comparing dev HEAD 4f020e5da4552fb254ce35db1adad5c63d68d09e and v1.6.3. Check dmesg output and the reported MI_LOCAL_DYNAMIC_TLS, MI_USE_CXX, and MI_SECURE configurations; done requires identifying why the webviews crash and establishing a verified fix.

Written by the indexing model from the issue text.