[NativeAOT] Use NativeLinker and invoke lld directly for linking

[sbomer]

Replace the clang Exec invocation with the LinkNativeAotSharedLibrary task
that calls ld.lld directly from the NDK. The task uses NativeLinker.cs which
handles response files, ABI-specific flags, and debug symbol stripping.

Compute NDK paths for CRT objects (crtbegin_so.o/crtend_so.o), compiler-rt
builtins, and libunwind. Pass system libraries (dl, z, log, m, c) and
library search paths explicitly.

Fix NativeLinker.cs quoting for soname with spaces, and guard against empty
runtime pack library directory.

Builds on #11148.
Contributes to #10697

[Copilot]

Pull request overview

This PR shifts NativeAOT shared-library linking to use the existing NativeLinker infrastructure (response files, ABI flags, symbol stripping) instead of invoking the NDK clang wrapper, and wires this into the NativeAOT MSBuild targets.

Changes:

• Extend NativeLinker with NativeAOT-focused linker options (export-dynamic, version scripts, linker scripts, debug section compression, extra -L paths, etc.).
• Add a new MSBuild task (LinkNativeAotSharedLibrary) that drives NativeLinker to produce a .so from ILC output + runtime/static archives.
• Update Microsoft.Android.Sdk.NativeAOT.targets to set NativeLib=static, enable unmanaged entrypoint exports, and run the new link step after LinkNative.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Utilities/NativeLinker.cs	Adds configurable linker options needed for NativeAOT and fixes argument quoting/guards.
src/Xamarin.Android.Build.Tasks/Tasks/LinkNativeAotSharedLibrary.cs	New task orchestrating NativeAOT .so linking via NativeLinker.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets	MSBuild integration: produce .a from ILC then link final .so via the new task, plus NDK CRT/rt/unwind inputs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/Xamarin.Android.Build.Tasks/Tasks/LinkNativeAotSharedLibrary.cs:140

• Directory.CreateDirectory (Path.GetDirectoryName (LinkerScript)!) can throw if LinkerScript is just a filename (no directory component), because Path.GetDirectoryName() would return null. Avoid the null-forgiving operator here and handle the null case explicitly (or use IntermediateOutputPath as the directory when none is provided).

		if (!LinkerScriptContent.IsNullOrEmpty () && !LinkerScript.IsNullOrEmpty ()) {
			Directory.CreateDirectory (Path.GetDirectoryName (LinkerScript)!);
			File.WriteAllText (LinkerScript, LinkerScriptContent);
		}

• Files reviewed: 3/3 changed files
• Comments generated: 4