[x86/Linux] Revert UMThkCallFrame-related changes (dotnet/coreclr#8434)

[x86/Linux] Revert UMThkCallFrame-related changes (dotnet/coreclr#8434)

* [x86/Linux] Revert UMThkCallFrame-related code

* [x86/Linux] Fix dangling 'TheUMEntryPrestub' reference

This commit re-enables GenerateUMThunkPrestub and its related code in
order to remove TheUMEntryPrestub reference.

* [x86/Linux] Re-enable several methods in StubLinkerCPU

This commit re-enables the following methods for x86/Linux:
 - StubLinkerCPU::EmitSetup
 - StubLinkerCPU::EmitComMethodStubProlog
 - StubLinkerCPU::EmitComMethodStubEpilog

In addtion, EmitComMethodStubEpilog is marked as NYI.

Commit migrated from https://github.com/dotnet/coreclr/commit/6ed21c52f25243b7cc1c64b19a47bbd4beb69314