[compiler-rt] [rtsan] Make sure rtsan gets initialized on mac (PR #100188)

[Chris Apple via llvm-commits]

cjappl wrote:

Two very small updates since last review:

1. Matching llvm style "If you use a braced initializer list when initializing a variable, use an equals before the open curly brace:"
```cpp
- static atomic_uint8_t rtsan_initialized{0};
+ static atomic_uint8_t rtsan_initialized = {0};
```

2. Got rid of the now-unused `IsInitialized` in favor of the `__rtsan_is_initialized` method, this was cruft from an earlier version of this review.

These two changes may be reviewed in the latest two commits, if anyone wants to see them in isolation.

I also rebased on latest main. I will likely merge this early next week

https://github.com/llvm/llvm-project/pull/100188