Make GCMemoryInfoData blittable

[MichalStrehovsky]

The runtime side makes assumptions about the layout of this class, but since the class is not blittable, the runtime is free to reorder the fields however it sees fit (StructLayout(LayoutKind.Sequential) only affects the managed layout of blittable classes - it's not documented to do anything to the managed layout of non-blittable classes).

This prevents improvements to managed layout such as #51416.

[jkotas]

Thanks

[MichalStrehovsky]

      Assert failure(PID 4696 [0x00001258], Thread: 764 [0x02fc]): Consistency check failed: Managed object size does not match unmanaged object size
      man: 0x114, unman: 0x118, Name: System.GCMemoryInfoData
      FAILED: size == expectedsize

Sigh. If I'm reading the tea leaves right, his is failing on 32bit platforms because the native side will try to align the beginning of GCMemoryInfoData so that it's 8-byte aligned (adding padding after the MethodTable pointer), but the managed side doesn't do that.

I wonder if the best course of action would be to just rewrite the FCall to take a reference to the first field of GCMemoryInfoData.

[jkotas]

@MichalStrehovsky PTLA at my suggested fix.

[MichalStrehovsky]

@MichalStrehovsky PTLA at my suggested fix.

Clever! Thank you!

I've re-kicked the failing CI leg because it looks like the known infra issue (collection was modified).