Support managed debugging when CET is enabled

[tommcdon]

This change adds out of process SetThreadContext support on Windows x64. The feature will be automatically enabled if the process is enabled for CET or with a configuration setting (useful for testing purposes). I've validated our mainline scenarios with the Visual Studio debugger, including breakpoints, stepping, mixed-mode debugging, and Edit-And-Continue (Hot Reload).

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Issue Details

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This change adds out of process SetThreadContext support on Windows x64. The feature will be automatically enabled if the process is enabled for CET or with a configuration setting (useful for testing purposes). I've validated our mainline scenarios with the Visual Studio debugger, including breakpoints, stepping, mixed-mode debugging, and Edit-And-Continue (Hot Reload).

Author:	tommcdon
Assignees:	-
Labels:	area-Diagnostics-coreclr
Milestone:	-

[noahfalk]

LGTM modulo comments inline

[tommcdon]

Test failures look unrelated to this PR and so merging