This change is adding DWARF (CFI) unwind information to the
hand-written assembly of the `__wine_syscall_dispatcher` function.
This enables unwinding through the dispatcher from the Linux stack
into (and through) the Windows stack.

The general idea is that the `syscall_frame` struct contains the
content of the callee-save registers before the function call
(in particular the stack pointer and the return address).  At any
point of the execution, we have a pointer into the `syscall_frame`
in $ebx, $ecx, $ebp, or $esp.

For the CFI codes the general idea is that we are defining the
computations of the callee-save registers based on the
`syscall_frame` using DWARF’s `breg` instruction, rather than
relative to CFA.