Improve detection of availability of GAC in GacFileResolver

[tmat]

Currently we check for absence of AssemblyLoadContext. Consider a better check (perhaps the presence of Assembly.GlobalAssemblyCache once CoreCLR mscorlib is cleaned up).

[gafter]

@tmat can you please assign this to the appropriate team? I don't know from the description what the problem is or which component is implicated.

[kennethy2]

I am not sure of the problem either we need a more in depth knowledge of the problem