win-x86: Replace "ret n" thunks with assembly code

[filipnavara]

Replace x86 COM "ret n" thunks with assembly code that emulates "ret [n]" for an "[n]" specified using a variable value stored in register.

[filipnavara]

The motivation is mainly to reduce the number of unnecessary usages of ExecutableWriteHolder for dynamic codegen. While the assembly code is not the prettiest due to restriction to use only single GP register it's still simple arithmetic instead of an indirect jump. Given the code around it the performance difference should be negligable. We still use ret so call-ret prediction for speculative execution should work just fine.

[jkotas]

There is an existing pattern used for this in this .asm file. Would it make sense to do the same here?

[jkotas]

/ba-g deadletter

[jkotas]

Thanks