Fix reading cpu cache size for Alpine(musl)

[mangod9]

Should resolve #41398. Will need to backport to .net 5.

[ghost]

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[janvorli]

LGTM, thank you!

[jkotas]

Thank you!

[jkotas]

@mangod9 Could you please also apply @am11 's suggestion: #41398 (comment)

[mangod9]

wondering if we might need to special case arm32 too, since I am seeing arm build failures, its not quite apparent from the errors whether new code is causing it, but seems suspicious.

[jkotas]

The ARM build errors are:

/__w/1/s/src/coreclr/src/pal/src/misc/sysinfo.cpp:578:12: error: no matching function for call to 'ReadMemoryValueFromFile'
        if(ReadMemoryValueFromFile("/sys/devices/system/cpu/cpu0/cache/index0/size", &size))
           ^~~~~~~~~~~~~~~~~~~~~~~
/__w/1/s/src/coreclr/src/pal/src/misc/sysinfo.cpp:514:1: note: candidate function not viable: no known conversion from 'size_t *' (aka 'unsigned int *') to 'uint64_t *' (aka 'unsigned long long *') for 2nd argument
ReadMemoryValueFromFile(const char* filename, uint64_t* val)
...

[mangod9]

Ah yes, I see them now in the raw build log. CI seems to suggest the native build was successful (the leg is green too), will follow up with Infra on this. Here are the final few lines from the build log

2020-08-28T21:42:45.5127898Z Repo successfully built.
2020-08-28T21:42:45.5129065Z Product binaries are available at /__w/1/s/artifacts/bin/coreclr/Linux.arm.Release
2020-08-28T21:42:45.5153650Z 

[jkotas]

The build infra issue is #34496

[mangod9]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/229731217

[adamsitnik]

Is there any way we could ensure that this problem never comes back? Like having a unit test that asserts that GetLogicalProcessorCacheSizeFromOS() returns more than zero? Or just a debug assert?

[mangod9]

@adamsitnik created #41708 for ensuring this doesnt regress.