Stack Limit as an Argument Purpose (#372)

* Initial approach.

* Move stack_limit check before opening the frame

* Account for GPRs and frame pointer in stack check

* Check stack_limit example.

* Remove stack_limit attribute code.

Amends #359

* fmt

cranelift/filetests/parser/preamble.clif

@@ -1,18 +0,0 @@
-test cat
-
-; Verify parsing of stack_limit.
-function %minimal(i64 vmctx) {
-gv0 = vmctx
-; Stack limit
-stack_limit = gv0
-
-ebb0:
-    trap user0
-}
-; sameln: function %minimal(i64 vmctx) fast {
-; nextln: gv0 = vmctx
-; nextln: stack_limit = gv0
-; nextln: 
-; nextln: ebb0:
-; nextln:     trap user0
-; nextln: }