jl_profile_atomic lock around jl_spawn to prevent macos profiling deadlock

[IanButterworth]

Fixes #43578

Thanks to @dnadlinger and @staticfloat for the pointers on this.

julia> using Profile

julia> @profile while true
              run(`pwd`)
              end
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# runs continuously, and signal handling still works, unlike on master

[IanButterworth]

Maybe this is doing something bad.. I just tried ProfileView with this PR and the Gtk window just shows black, which doesn't happen on master

[IanButterworth]

The last commit doesn't fix the Gtk issue, but it seems like a better approach

[giordano]

I presume the gtk issue isn't JuliaGraphics/Gtk.jl#590?

Edit: it isn't clear to me whether the bug was specific to the M1 or not, in any case it should have been fixed by JuliaPackaging/Yggdrasil#4150

[IanButterworth]

Yeah! That fixes ProfileView. Thanks.

Unfortunately more testing of this PR has shown it's interfering with execution time substantially

julia> t = Timer(10)

julia> @profile while isopen(t)
       run(`pwd`)
       sleep(0.1)
       end
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julia>

So run only gets to acquire the profiling lock 3 times in 10 seconds

[IanButterworth]

Switching the lock nesting fixes that ^

[vtjnash]

superseded by #43701