Design portable tail calls with helpers (dotnet/coreclr#8596)

Design portable tail calls with helpers (dotnet/coreclr#8596)

* Design portable tail calls with helpers

Currently we don't handle non-trivial cases of tail calls on Unix due to the non-portable and
complex way it is implemented. This pull request describes a design of a portable way of
handling these cases so that it can be used uniformly over all our jitted and AOT (ahead of time)
platforms.

Commit migrated from https://github.com/dotnet/coreclr/commit/647e7f1f39040fbd7e77d1d7cc56c22deb7ec2a9