To have a convenient place to add a CFI_ENDPROC call.

Signed-off-by: Edwin Török <edwin@etorok.net>

This is needed for backtraces to function in GDB.

CFI_{START,END}PROC is added to FUNCTION/END_FUNCTION.
.Ltrap_handler needs its own CFI_STARTPROC because it doesn't use
FUNCTION.

The rules for CFI_ADJUST are fairly simple: whenever the `sp` is
manipulated add a CFI_ADJUST right after it with an opposite sign.
This allows the unwinder to always find the frame even when the sp
gets moved multiple times.

CFI_OFFSET is used to tell the unwinder where the various registers are
saved on the stack. At a minimum we need to tell it where to find `ra`,
so that it can continue unwinding the stack.
The value for CFI_OFFSET is calculated by undoing the adjustment
made by CFI_ADJUST (so we get the value of the sp from the frame),
and then adding the same offset use by the STORE instruction.

Signed-off-by: Edwin Török <edwin@etorok.net>

Needed for stacktraces to work in a debugger.
cfi_startproc/cfi_endproc marks the boundaries of a function.

adjust_stack_offset updates the offset between the stackpointer and the
frame as the stackpointer is adjusted the offset is adjusted in opposite
direction.
Every `addi sp` needs a corresponding adjust_cfa_offset.
This is not done for tailcalls where other backends have a comment about
undoing CFA adjustments because the function may continue.

cfi_offset is for telling the unwinder where to find register on the
stack, at a minimum we need to tell it about the location of `ra`.

Note that there is a difference here: in OCaml code it is at c-8,
whereas in caml_call_gc it is at c-16
(c-8 is unused gap due to alignment).

This change was modeled after arm64/emit.mlp

Signed-off-by: Edwin Török <edwin@etorok.net>