[compiler-rt] [rtsan] Support legacy pthread_cond variables (PR #152947)

[Chris Apple via llvm-commits]

cjappl wrote:

I just hopped off a call with @adrew0809 and we confirmed this fixes the issue!

The surprising part is his code no longer crashes when the flag is omitted, or when it is it's default value (false)

```
./a.out # no crash
RTSAN_OPTIONS='legacy_pthread_cond=false` ./a.out # no crash
RTSAN_OPTIONS='legacy_pthread_cond=true` ./a.out # CRASH
```

I'm trying to reason about why this might be. Maybe it has something to do with the PTHREAD_ABI_BASE? I'm not entirely sure.


Considering this fixes the issue, I'm inclined to merge. Any objections @fmayer or @davidtrevelyan ? 

https://github.com/llvm/llvm-project/pull/152947