Introduce a diagnostic switch to turn off the use of file mapping in PE loader.

[VSadov]

Introducing unsupported/internal DOTNET_PELoader_DisableMapping switch.

The switch is available in both debug and release builds.
Setting DOTNET_PELoader_DisableMapping to not 0 will force the fallback paths in PE loader where file sections are copied to preallocated memory ranges instead of being mapped.

We needed to turn off file mapping in a few scenarios recently when narrowing down the root cause of bugs.
Without a switch, turning off the file mapping is nontrivial and especially so in a singlefile scenario.

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Issue Details

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We needed this in a few scenarios recently when narrowing down the root cause of bugs. Without a switch, turning off the file mapping is nontrivial and especially so in a singlefile scenario.

=== in progress.

Need to:

• test manually to make sure the switch really does what it is supposed to do.
• make the default state "off". (run tests with "on" to make sure it is ok).

Author:	VSadov
Assignees:	VSadov
Labels:	area-AssemblyLoader-coreclr, area-Single-File, area-VM-coreclr
Milestone:	-

[VSadov]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[VSadov]

CC: @tommcdon

[VSadov]

close/reopen to ustuck the CLA check

[janvorli]

LGTM, thank you!

[VSadov]

Thanks!!