Change Backtrace::enabled atomic from SeqCst to Relaxed

[dtolnay]

This mirrors rust-lang/rust#92139.

The atomic is not synchronizing anything outside of its own value, so we don't need the Acquire/Release guarantee that all memory operations prior to the store are visible after the subsequent load, nor the SeqCst guarantee of all threads seeing all of the sequentially consistent operations in the same order.

Using Relaxed reduces the overhead of Backtrace::capture() in the case that backtraces are not enabled.