[6.0][Runtime] Don't emit signposts until the system is ready.

[mikeash]

Cherry-pick #73639 to release/6.0.

Emitting a signpost for the first time can trigger lazy setup of the logging system, and doing this in the wrong context can cause deadlocks. Check to see if the logging system is already set up, and only emit signposts if it has been to avoid triggering this.

As it's hard to determine if the "is it set up?" function is available in the SDK we're building against, only do this in OS builds, as it's not particularly useful in local builds.

rdar://124620772
(cherry picked from commit dd24d8e)

[mikeash]

@swift-ci please test

[mikeash]

Explanation: We want to refrain from emitting signposts until the logging system is set up by something else, because triggering setup in certain contexts can lead to deadlocks.
Original PR: #73639
Reviewed by: @ktoso
Risk: Low. We make one additional call before emitting a signpost, and that call just checks an atomic global.
Resolves: rdar://124620772
Tests: This code runs extensively in pretty much all executable tests and when running programs normally. Tested manually to ensure that the new call gets made and behaves the way it's expected to.