Consider using memfd_create instead of shm_open in MemoryMappedFile if it's available

[stephentoub]

When this code was originally written, memfd_create didn't exist. Now that it does, we can consider using it instead of shm_open if it's available (e.g. on Linux)

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Issue Details

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When this code was originally written, memfd_create didn't exist. Now that it does, we can consider using it instead of shm_open.

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO
Milestone:	9.0.0

[gfoidl]

From Chromium source:

// Check kernel version supports memfd_create(). Some older kernels segfault
// executing memfd_create() rather than returning ENOSYS (b/116769556).

But I don't know if Linux kernels older than 3.17 are still in use / supported or meet the .NET baseline (can't find that info right now :-()

PS: I saw the "if it's available", just wanted to point this out so that we don't run in strange troubles.

[adamsitnik]

cc @tmds