Respect memory constraints during init.

[maleadt]

I'm not sure #24617 should have been closed, as uv_get_total_memory doesn't change when running with, e.g., docker run --memory 3G. Instead, libuv now has uv_get_contrained_memory, which we should probably respect when initializing the heap.

I think this is the cause for some GPU CI issues, where docker kills the process once we exceed the memory limit (i.e. we don't get a nice OOM error). That happens often, I assume because the GC doesn't know about the actual memory limit.

Like nodejs/node#27508

[maleadt]

We should probably expose this somewhere in Sys too (or should Sys.total_memory report the constrained memory?), but I wanted to get this in 1.5 if deemed OK (or a patch release) so I left that feature out for now. This should be a strict improvement that only matters in a non-default setting, e.g., without doing anything:

julia> ccall(:uv_get_total_memory, Int64, ()) |> Base.format_bytes
"31.249 GiB"

julia> ccall(:uv_get_constrained_memory, Int64, ()) |> Base.format_bytes
"8192.000 PiB" (around typemax(Int64))