[Build] Add per-file up-to-date check in CompileNativeAssembly

[davidnguyen-tech]

Summary

Two related changes that together skip _CompileNativeAssemblySources on no-type-change incremental CoreCLR builds:

1. Stabilize the typemap LLVM-IR output so that builds with no type changes produce a byte-identical typemaps.<abi>.ll, letting Files.CopyIfStreamChanged preserve its mtime → MSBuild's target-level Inputs/Outputs check skips _CompileNativeAssemblySources entirely.
2. Per-file llc up-to-date check as defense-in-depth: when the target does run (because some .ll did change), skip individual files whose .o is already up to date.

Problem

On incremental CoreCLR builds, _CompileNativeAssemblySources was rebuilding every .ll → .o even when nothing relevant had changed:

• TypeMappingDebugNativeAssemblyGeneratorCLR embedded an mvid_hash and emitted entries in hash-bucket order, so any assembly rebuild flipped bytes inside typemaps.<abi>.ll even when the type set was identical. CopyIfStreamChanged then updated mtime, which busted the target's Inputs/Outputs check.
• <CompileNativeAssembly> (the task) had no per-file granularity — once the target ran, it called llc for every source even if .o was newer than .ll.

Fix

Utilities/TypeMappingDebugNativeAssemblyGeneratorCLR.cs

• Remove mvid_hash from emitted typemap entries.
• Sort entries by managed type name so output is deterministic across runs.

Tasks/CompileNativeAssembly.cs

• In RunAssembler(), compare File.GetLastWriteTimeUtc of the output .o against the input .ll. If .o exists and is ≥ .ll, skip the llc invocation and log:
  [LLVM llc] Skipping '<x>.ll' because '<x>.o' is up to date

These changes also benefit _CompileNativeAssemblySources callers in Microsoft.Android.Sdk.NativeRuntime.targets and Microsoft.Android.Sdk.NativeAOT.targets.

Verification

Verified end-to-end on Samsung SM-G960F (R58Y30J85VV) against dotnet new maui -sc (MauiVersion 11.0.0-preview.4.26221.9, net11.0-android, CoreCLR, arm64-v8a, Debug):

• Typemap determinism — typemaps.arm64-v8a.ll byte-identical and mtime-preserved across a no-type-change edit.
• Target skip — head-to-head binlog probes:
  • PR side: Skipping target "_CompileNativeAssemblySources" because all output files are up-to-date
  • main side: Building target "_CompileNativeAssemblySources" completely
• Per-file llc skip — exercised by the integration test (see below).
• App still works — deploys, launches, MainActivity resolves through the typemap name-iterate path.
• Wall-time — 5 runs each side, ~1.0–1.6 s saved on this minimal app. Savings scale with typemap size; expect more on real apps.

Testing

CompileNativeAssemblySourcesSkipsUnchangedFiles (in IncrementalBuildTest.cs) verifies both behaviours in one build:

• Forces typemap regeneration by appending a new [Activity]-attributed TypeMapProbeActivity class — GenerateJavaStubs adds a JCW, so typemaps.<abi>.ll content + mtime genuinely change.
• Asserts _CompileNativeAssemblySources is not skipped at the target level.
• Asserts the per-file skip log line fires for environment.<abi>.ll (which is unaffected by user types and so stays up-to-date).

Test runs at Detailed verbosity so LogDebugMessage (Low importance) is captured.