Add ICU App-Local support on Mono

[safern]

This adds AppLocalIcu switch support on Mono. I also refactored GlobalizationMode to share the sources in between Mono and CoreCLR to avoid duplicating all the AppLocal and UseNls switches logic.

cc: @jkotas @thaystg @akoeplinger @tarekgh @ericstj

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Tagging subscribers to this area: @tarekgh, @safern, @krwq
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[vargaz]

The linker will need a bool method which it can transform to false to disable ICU support at app build time.

[jkotas]

Does the existing GlobalizationMode.get_Invariant property work for this? (It would need to be tranformed to true.)

[vargaz]

It does.

[safern]

FWIW, we're still doing something similar:

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.Unix.cs

Lines 17 to 32 in b0ed2bd

	bool invariantEnabled = GetInvariantSwitchValue();
	if (!invariantEnabled)
	{
	if (TryGetAppLocalIcuSwitchValue(out string? icuSuffixAndVersion))
	{
	LoadAppLocalIcu(icuSuffixAndVersion, suffixWithSeparator: true);
	}
	else if (Interop.Globalization.LoadICU() == 0)
	{
	string message = "Couldn't find a valid ICU package installed on the system. " +
	"Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.";
	Environment.FailFast(message);
	}
	}
	return invariantEnabled;
	}

[safern]

@vargaz does this look good, or is there any action I should take?

[safern]

I merged this, but if something else need to happen here, I'm happy to put up a follow up PR.

[CoffeeFlux]

Runtime changes look good to me, thanks!

[safern]

@tarekgh @jkotas S.P.CoreLib changes look good?

[jkotas]

@tarekgh @jkotas S.P.CoreLib changes look good?

Yes