[NativeAOT-LLVM] Optimize always-throwing functions and cctors for size

[SingleAccretion]

Initially I was looking at utilizing the Cold attribute to mark always-throwing calls, but it turns out marking cold things cold is a code size pessimization due to missing CSEs, so this is just a very minimal change to mark defined functions which we know will be cold as "optsize".

Diffs are also minimal, but nevertheless positive:

Summary of Code Size diffs:
(Lower is better)

Total bytes of base: 3247941
Total bytes of diff: 3247777
Total bytes of delta: -164 (-0.01% % of base)
Average relative delta: -6.47%
    diff is an improvement
    average relative diff is an improvement

Top method improvements (percentages):
         -45 (-7.77% of base) : 1000.dasm - S_P_CoreLib_Internal_Runtime_IDynamicCastableSupport__IDynamicCastableGetInterfaceImplementationFailure
         -33 (-6.85% of base) : 1002.dasm - S_P_CoreLib_System_Text_DecoderExceptionFallbackBuffer__Throw
         -33 (-6.45% of base) : 1001.dasm - S_P_CoreLib_System_Text_EncoderExceptionFallbackBuffer__Fallback_0
         -53 (-4.82% of base) : 1003.dasm - S_P_CoreLib_System_Number___cctor

4 total methods with Code Size differences (4 improved, 0 regressed)

[SingleAccretion]

@dotnet/nativeaot-llvm