Epic: Replace Crossgen with Crossgen2

[mangod9]

As part of .NET 5 we introduced Crossgen2 to pre-compile assemblies. As part of .NET 6 Crossgen2 will replace Crossgen1 within the runtime and SDK. This contributes towards this Theme: dotnet/core#5438

Functionality

• Migration-blocking bugs fixed / triaged
• x86 / arm32 support
• Apple Silicon parity with Crossgen
• PDB generation [Tomas]
• Cross-compilation support (arm64 -> arm32, x64 -> x86) [David]
• Version resiliency testing [Simon]
• SDK support for cross-architecture compilation [Anton]
• Add Arcade support for building R2R images with CG2 [Anton]
• Perfmap generation and integration into perfcollect
• PGO block count handling
• Support using Crossgen2 for compiling System.Private.CoreLib in CoreCLR build Support using Crossgen2 for compiling System.Private.CoreLib in CoreCLR build #43810
• More options for performing JIT. AB#1244349

Testing

• Crossgen2 tests pass > 99%
• Crossgen2 outerloop tests pass > 99%
• Stress tests passing > 99%
• Set of methods compiled by CG2 matches CG1 (via R2RDump), Use R2RDump to validate that the set of methods compiled with CG2 matches CG1 #50124
• At runtime, set of methods executed as R2R matches CG1 (via ETW), Use ETW to verify that runtime execution runs the same R2R methods for CG1 vs. CG2 #50125
• x86 / arm32 Testing (improved CI testing from minimal)

Performance

• Small file compilation performance improvements
• Define real-world code sign-off criteria (WPF, Powershell startup, ASP.NET startup / throughput, eg)
• Compiler throughput >= Crossgen1
• R2R image sizes <= Crossgen1 [Simon]

Rollout (and Exit Criteria)

• S.P.C AOT'd with CG2 [Tomas]
• Add library test CI run using CG2-compiled framework, Implement support for library tests using CG2-compiled framework and add the corresponding CI run #50127
• Libraries AOT'd with CG2 (staggered by architecture?)
• Switch SDK default to CG2
• Remove crossgen from runtime repo

[mangod9]

I am closing this userstory since we have specifically completed the following (we will track some of the remaining work as separate issues):

Switch SDK default to CG2
Remove crossgen from runtime repo

Thanks @dotnet/crossgen-contrib for getting us here :).