[wasm] unify Interop.GetHostName

[pavelsavara]

From comment

nit: it feels slightly wrong doing this special case in the Interop.GetHostName.cs managed binding layer, but calling into native code just for getting a constant string feels wrong too.

Btw. after looking at it some more I think we have an issue on Browser: we do use emscripten's gethostname() which returns the string emscripten and we just paper over it here:

runtime/src/libraries/System.Private.CoreLib/src/System/Environment.Browser.cs

Lines 13 to 15 in 4cdbfdc

	// In the mono runtime, this maps to gethostname, which returns 'emscripten'.
	// Returning the value here allows us to exclude more of the runtime.
	public static string MachineName => "localhost";

But Interop.Sys.GetHostName() is used in more places and those would get the "wrong" hostname on Browser. I think I'd be in favor of unifying this and adding the special case for Browser here too.

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[pavelsavara]

cc @akoeplinger