misc: Replace references to IntPtr/UIntPtr with nint/nuint + code cleanups.

src/ARMeilleure/CodeGen/Arm64/HardwareCapabilities.cs

src/ARMeilleure/CodeGen/CompiledFunction.cs

src/ARMeilleure/CodeGen/RegisterAllocators/LiveInterval.cs

src/ARMeilleure/CodeGen/RegisterAllocators/LiveRange.cs

src/ARMeilleure/Common/AddressTable.cs

src/ARMeilleure/Common/ArenaAllocator.cs

src/ARMeilleure/Common/EntryTable.cs

src/ARMeilleure/Common/NativeAllocator.cs

src/ARMeilleure/IntermediateRepresentation/IntrusiveList.cs

src/ARMeilleure/IntermediateRepresentation/MemoryOperand.cs

src/ARMeilleure/IntermediateRepresentation/Operation.cs

src/ARMeilleure/Memory/IJitMemoryBlock.cs

src/ARMeilleure/Memory/IMemoryManager.cs

src/ARMeilleure/Memory/ReservedRegion.cs

src/ARMeilleure/Native/JitSupportDarwin.cs

src/ARMeilleure/Signal/NativeSignalHandlerGenerator.cs

src/ARMeilleure/Signal/TestMethods.cs

src/ARMeilleure/Signal/WindowsPartialUnmapHandler.cs

src/ARMeilleure/State/ExecutionContext.cs

src/ARMeilleure/State/NativeContext.cs