backport memory pressure callback to 1.9

[d-netto]

PR Description

Backports the memory pressure callback to 1.9.

Requirements for merging:

• I have opened an issue or PR upstream on JuliaLang/julia: Page based heap size heuristics JuliaLang/julia#50144
• I have removed the port-to-* labels that don't apply.
• I have opened a PR on raicode to test these changes: I believe it's not applicable, since we're not using it in raicode, but happy to introduce a few tests in raicode.

[d-netto]

MWE that should give some guidance on how to use:

using Printf

function mem_pressure_callback()
        @printf "memory pressure callback called\n"
end

callback_as_cfunction = @cfunction(mem_pressure_callback, Cvoid, ())

ccall(:jl_gc_set_cb_notify_gc_pressure, Cvoid, (Ptr{Cvoid}, Cint), callback_as_cfunction, true)

GC.gc()

Testing the callback with a low --heap-size-hint :

./julia --heap-size-hint=40k mwe.jl 
memory pressure callback called

--heap-size-hint=500M (or any large value) doesn't output anything, as expected.