Mmap fix on system with 64k pagesize.

[green-br]

Not sure if this is the best approach but seemed to allow Mmap test to pass. Seems the pagesize was being set to value at build-time rather than being re-evaluated at runtime. Running Mmap tests on system with 64k pagesize, Mmap would still think its on a system with 4k pagesize and therefore fail.

[giordano]

@vtjnash is

Suggested change

	PAGESIZE = Int(Sys.isunix() ? ccall(:jl_getpagesize, Clong, ()) : ccall(:jl_getallocationgranularity, Clong, ()))
	const PAGESIZE = OncePerProcess{Int}() do
	Int(Sys.isunix() ? ccall(:jl_getpagesize, Clong, ()) : ccall(:jl_getallocationgranularity, Clong, ()))
	end

the right way to do this on master?

[giordano]

But then all references to PAGESIZE should be changed to PAGESIZE(). Apparently there are some packages which were using this internal symbol assuming it was a constant, and we'll need to update them: https://juliahub.com/ui/Search?type=code&q=Mmap.PAGESIZE

[vtjnash]

I'm not certain you need the overhead of OncePerProcess to memoize the read of the getpagesize variable

[giordano]

So this should just be a regular function?

[giordano]

I opened #59712 as an alternative to this PR, to create a pagesize function.

[green-br]

Thanks for taking a look - I was looking eleswhere in the code and the Sys module which seems to use __init__ function to set similar type of variables. e.g. Sys module

Made me think whether PAGESIZE should be in Sys to keep these sorts of settings in the same location.

[vtjnash]

Agreed, Sys.__init__ makes sense, then just needs using Sys: PAGESIZE here

[green-br]

Attempted to test this out or at least draft it.