Adjust relative offsets in UMThunkStub (dotnet/coreclr#4888)

Adjust relative offsets in UMThunkStub (dotnet/coreclr#4888)

* Adjust relative offsets in UMThunkStub

This commit updates the relative offsets inside UMThunkStub according to the recent changes in
UMThunkStub (for stack unwinding).

* Uses 'UMThunkStub_StackArgsSize' instead of 'UMThunkStub_StackArgsOffset'

This commit revises 'UM2MThunk_WrapperHelper' to use 'UMThunkStub_StackArgsSize'
instead of 'UMThunkStub_StackArgsOffset'.

Commit migrated from https://github.com/dotnet/coreclr/commit/7fe0fef10a3c9654673aff5366d97f3dc74a68e5