Currently `avo` uses `BP` as a standard general-purpose register. However, `BP` is used for the frame pointer and should be callee-save. Under some circumstances, the Go assembler will do this automatically, but not always. At the moment `avo` can produce code that clobbers the `BP` register. Since Go 1.16 this code will also fail a new `go vet` check.
This PR provides a (currently sub-optimal) fix for the issue. It introduces an `EnsureBasePointerCalleeSaved` pass which will check if the base pointer is written to by a function, and if so will artificially ensure that the function has a non-zero frame size. This will trigger the Go assembler to automatically save and restore the BP register.
In addition, we update the `asmdecl` tool to `asmvet`, which includes the `framepointer` vet check.
Updates #156
This makes it easier to debug avogen: when you emit invalid syntax, you can inspect the generated file to determine what went wrong, instead of having only gofmt's error to work with.
