Epic: Enable W^X

[janvorli]

We plan to enable W^X in .NET 6 on all platforms and architectures. The goal is to get rid of all memory page mappings that are both executable and writeable at the same time.

• Implement low level allocator of executable memory that will enable double mapping of executable pages with RX and RW protection to different memory locations.
• Modify code heaps in the runtime to use that allocator and provide separate RW and RX mappings and also mechanism for releasing of the RW mapping for their clients.
• Modify stub generating code to use a separate RW mapping for writing the code and to release the RW mapping after that.
• Modify JIT to a separate RW mapping for writing the code and to release the RW mapping after that.
• Modify GC write barrier switching code to use separate RW mapping for rewriting the barrier code.
• Integrate the changes with the existing W^X support for Apple M1.
• Performance validation for all architectures
• Enable by Default

Postponed to post 6.0:

• Modify selected stubs to use static code in RX pages with stub specific data read indirectly from separate RW pages. An example of such a stub is the UMEntryThunk. The benefit is that there is no need to update the code after the initial generation and thus no creation / removal of RW mapping for code is needed after the initialization.

[mangod9]

@JulieLeeMSFT @sandreenko @sdmaclea FYI

[mangod9]

Closing since this is now complete. No known bugs are pending.