Implement getFunctionEntryPoint for runtime-async R2R compilation

[jtschuster]

Runtime-async methods call into AsyncHelpers (CaptureContexts, RestoreContexts, etc.) via method handles returned by getAsyncInfo. Previously, crossgen2's getFunctionEntrypoint created a delay-load import for these calls, but only updateEntryPointForTailCall (PR dotnet#56669) actually had jumpable thunk support, so when the JIT called this getFunctionEntryPoint for jmp instructions, those calls failed (see callconv3_d jit test).

This change removes the CORINFO_HELP_ASYNC_* JIT helper infrastructure and instead routes all async helper calls through getFunctionEntrypoint with standard (non-jumpable) delay-load imports.

JIT changes (async.cpp):

• Replace gtNewHelperCallNode(CORINFO_HELP_ASYNC_*) with gtNewCallNode(CT_USER_FUNC, methHnd) for all 7 async helper call sites (CaptureContexts, RestoreContexts, CaptureContinuationContext, CaptureExecutionContext, RestoreContextsOnSuspension, RestoreExecutionContext)
• Call getFunctionEntrypoint + setEntryPoint on each call node under FEATURE_READYTORUN so fgMorphArgs adds the R2R indirection cell arg (x11 on ARM64, found from return address on x64).

JIT changes (codegenxarch.cpp):

• In GT_JMP epilog, call updateEntryPointForTailCall to upgrade the import to a jumpable thunk, which will cause the JIT to load rax with the indirection cell address. This is necessary for jmps targeting delayload imports because jmp has no return address for the standard x64 thunk to decode. This is similar to what was necessary for tail calls.

Crossgen2 changes (CorInfoImpl.ReadyToRun.cs):

• Use regular DelayLoadMethodImport for calls to getFunctionEntrypoint. The JIT's GT_JMP codegen calls updateEntryPointForTailCall when needed to get a jumpable import.

NativeAOT scanner fix (ILImporter.Scanner.cs):

• Fix incorrect method names (AsyncCaptureExecutionContext -> CaptureExecutionContext, etc.) in dependency tracking.

R2RDump fix (CoreDisTools.cs):

• Handle ARM64 BL instructions targeting async resumption stubs, which are separate RuntimeFunctions not listed in the target method's collection. Replace Debug.Fail for now until we decide we want to move forward with this design.

Removed CORINFO_HELP_ASYNC_* helpers from corinfo.h, jithelpers.h, readytorun.h, readytorunhelpers.h, utils.cpp, ReadyToRunConstants.cs, CorInfoHelpFunc.cs, JitHelper.cs, ReadyToRunSignature.cs, and CorInfoImpl.RyuJit.cs.