[Mono] Intrinsify Unsafe API's

[fanyang-mono]

Fixes #101495

The implementation of Unsafe.BitCast was extracted from #103915

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[xtqqczze]

Make sure to add comments to Unsafe.cs, e.g. // Mono:SubtractByteOffset as below:

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/Unsafe.cs

Lines 25 to 31 in 19f0385

	// CoreCLR:METHOD__UNSAFE__AS_POINTER
	// AOT:AsPointer
	// Mono:AsPointer
	[NonVersionable]
	[CLSCompliant(false)]
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static void* AsPointer<T>(ref T value)

[tannergooding]

The Add/Subtract and Read/Write changes LGTM

BitCast also LGTM, but I'm probably not the right person to review that since I wrote some of the code in a previous prototype

[fanyang-mono]

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/Unsafe.cs

This seems to be a very fragile way of keeping track which methods are intrinsified by each runtime. We probably should come up a better way of doing it.