[ObjWriter/Mach-O] Mark non-global symbols as .private_extern to match ELF

[filipnavara]

Extracted from #106224

.private_extern is the logical equivalent of .hidden+.global in ELF. We already emit those flags in ELF, so do it in Mach-O too.

Documentation for .private_extern:

It's used to mark symbols with limited visibility. When the file is fed to the static linker, it clears the N_EXT bit for each symbol with the N_PEXT bit set. The ld option -keep_private_externs turns off this behavior.

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[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop, runtime-extra-platforms

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[MichalStrehovsky]

Cc @ivanpovazan @dotnet/jit-contrib @dotnet/ilc-contrib for Apple-specific ObjWriter change. Looks reasonable to me but don't know much about Apple stuff.

[jkotas]

LGTM

@filipnavara I assume that we do not need to backport this fix to .NET 9. Correct?

[filipnavara]

I assume that we do not need to backport this fix to .NET 9. Correct?

Correct. I don't see a need to backport it at this point.

[ivanpovazan]

LGTM, thank you!