Implement GcInfo v2

Implement GcInfo v2
Ref dotnet/coreclr#4379

This change implements GcInfo version 2 for all platforms that use the
GcInfo library (all architectures other than X86).

Changes are:
1) Defines ReturnKind enumeration for all platforms
2) Change the GcInfo encoder library to encode the ReturnKind and ReversePInvokeFrame slot
3) Change the CM's GcInfo decoder to encode the ReturnKind and ReversePInvokeFrame slot for GCINFO_VERSION 2
4) Some corrections to GCINFO_MEASUREments
5) Changes to RYU Jit to provide the correct information to the encoder
6) Changes to the VM to use the ReturnKind information while hijacking a thread
   - If ReturnKind is available from GcInfo, new hijack routines are used
   - Otherwise, fall back to old method (for compatibility)
7) Rework and simplify the thread hijack routines by scanning HijackFrames directly for gcroots
8) Supporting code to implement the above features.

Returning Structs in multiple registers

Hijacking for StructInRegs is currently only implemented for
Unix SystemV ABI Multi-reg struct returns. However, the hijack-workers that use
ReturnKind are ready to handle other platforms (ex: ARM/ARM64 Windows)
once the corresponding HijackTripThread() assembly routines are defined.

The New feature flag: FEATURE_MULTIREG_RETURN is set for platforms where a struct value
can be returned in multiple registers [ex: Windows/Unix ARM/ARM64, Unix-AMD64]
FEATURE_UNIX_AMD64_STRUCT_PASSING is a specific kind of FEATURE_MULTIREG_RETURN
specified by SystemV ABI for AMD64

Compatibility with other JITs

- All new GCInfo generated by RYU Jit is in GcInfo version 2
- All Ngen images must be regenerated with the new GcInfo version.
- Ready-to-run images with old GcInfo will continue to work.
- Jit64/X64 uses the GcInfo library, so it generates GcInfo version 2.
  However, it doesn't (yet) provide the data to encode the correct ReturnKind
  Similar is the case for ARM32 code running on JIT32, and any other JITs
  that may be using GcInfo library but not yet modified to use the new API.
  So, compatibility is achived using RT_Unset flag.

When ReturnKind is RT_Unset, it means that the JIT did not set
the ReturnKind in the GCInfo, and therefore the VM cannot rely on it,
and must use other mechanisms (similar to GcInfo ver 1) to determine
the Return type's GC information.

Implement GC root scanning for Hijack-frames

This change implements GCScanRoots() method for Hijacke-frames
based on the ReturnKind information available from the GcInfo.
If the exact ReturnKind is not available in the GcInfo, the
thread-suspension logic will compute the ReturnKind based on
the method-signature.

As a result of this change, several hijack-helpers in the VM
are cleaned up. There's only one implementation of HijackWorker()
to handle all returnKinds.

This change also simplifies the thread-hijack logic by using a
single assembly helper OnHijackTripThread() in most cases.
The only other helper used is for X86 floating point return values
for save/restoring the top of the FP stack.

ARM64

Only GcIndfo v2 is reliably supported for ARM64 platform.
The changes to thread-hijack mechanism fixes dotnet/coreclr#6494 for ARM64.

No measurable change in JIT throughput, performance or native-image
size from this change.

Commit migrated from https://github.com/dotnet/coreclr/commit/d302e64bcb4ea445f6702c529f8a964df20ab494