Add NativeAOT support for armel

[ashaurtaev]

This PR adds NativeAOT support for linux-armel (ARM soft-float)

Previously crossgen2 and ILCompiler were not built for armel and the NativeAOT runtime pack was missing for this architecture.

Changes:

• armel is now a separate platform in NativeAOT (not mapped to arm) with ARM_SOFTFP for soft-float ABI
• Added runtime assembly stubs: GcProbe, InteropThunks, UniversalTransition, StackFrameIterator
• Registered linux-armel in RID lists for runtime packs, ILCompiler, crossgen2, and apphost
• Enabled SDK tools and host.native for armel in default CoreCLR subsets

cc @dotnet/samsung

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[jkotas]

Suggested change

<DefaultCoreClrSubsets Condition="'$(TargetRid)' == 'linux-armel'">clr.native+clr.corelib+clr.tools+clr.nativecorelib+clr.packages+clr.nativeaotlibs+clr.crossarchtools+host.native</DefaultCoreClrSubsets>

Unnecessary - same as the default on previous line

[jkotas]

Is the thing you want to enable really linux-armel with glibc; or are you actually trying to enable native AOT for linux-bionic-arm?

[jkotas]

Build breaks in Tizen CI leg

[am11]

Is the thing you want to enable really linux-armel with glibc; or are you actually trying to enable native AOT for linux-bionic-arm?

Android/bionic do not support armel.

Build breaks in Tizen CI leg

Since linux-armel isn't officially supported, it could be added to bootstrap community platform next to:

runtime/eng/pipelines/runtime.yml

Line 695 in 605d2f4

- freebsd_x64

[jkotas]

Android/bionic do not support armel.

Android/bionic use armel ABI on arm by default. This detail is not visible in the RID - the RIDs are still linux-bionic-arm and android-arm. Armel ABI is enabled automatically when targeting Android/bionic arm.

[jkotas]

Since linux-armel isn't officially supported, it could be added to bootstrap community platform next to

I assume that @ashaurtaev is actually interested in targeting Tizen that we have a CI leg for already.

[am11]

From https://developer.android.com/ndk/guides/abis.html?utm_source=chatgpt.com

For historical reasons, this ABI uses -mfloat-abi=softfp causing all float values to be passed in integer registers and all double values to be passed in integer register pairs when making function calls. Despite the name, this only affects the floating point calling convention: the compiler will still use hardware floating point instructions for arithmetic.

[am11]

we have a CI leg for already.

PR is enabling host subset, that's the one which causes issues when official nuget isn't present. Previously it was getting linux-arm LKG packages.

[jkotas]

These structures have to be in sync with CoreCLR w/ JIT. We do not have these fine-grained ifdefs in CoreCLR. https://github.com/dotnet/runtime/blob/main/src/coreclr/vm/arm/cgencpu.h

These are all slow paths. It is very minor inefficiency to save the extra floating-point registers with armel even though it is not strictly necessary. I think it would be simpler to avoid these ifdefs and accept this minor inefficiency. (Otherwise, you need a bunch more places that need these ifdefs to ensure everything is in sync.)