[mono] Fix linux-x64 AOT-llvm perf job

[matouskozak]

Based on the findings discussed in dotnet/performance#3338 Mono Linux x64 performance job wasn't doing AOT-llvm. In this PR we are adding /p:MonoAOTEnableLLVM=true and /p:MonoEnableLLVM=true build flags that are needed for Ubuntu x64 to do AOT with LLVM backend.

Additionally, we are adding mono-aot-cross --llvm --version command to the Helix job for future verification of enabled LLVM.

Changing llvm:false tag to llvm:true for x64 AOT Perf jobs. (Arm64 change will follow later for .NET 9)

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cc: @kotlarmilos @directhex @LoopedBard3

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Issue Details

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Based on the findings discussed in dotnet/performance#3338 Mono Linux x64 performance job wasn't doing AOT-llvm. In this PR we are adding /p:MonoAOTEnableLLVM=true and /p:MonoEnableLLVM=true build flags that are needed for Ubuntu x64 to do AOT with LLVM backend.

Additionally, we are adding mono-aot-cross --llvm --version command to the script responsible for creating AOT directory for future verification of enabled LLVM.

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cc: @kotlarmilos @directhex @LoopedBard3

Author:	matouskozak
Assignees:	matouskozak
Labels:	area-Infrastructure-mono
Milestone:	-