pass: ensure frame pointer register is saved (#174)

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

tests/alloc/gp8/gp8.s

@@ -3,7 +3,7 @@
 #include "textflag.h"
 
 // func GP8() uint8
-TEXT ·GP8(SB), NOSPLIT, $0-1
+TEXT ·GP8(SB), NOSPLIT, $8-1
 	MOVB $0x01, AL
 	MOVB $0x02, CL
 	MOVB $0x03, DL

tests/alloc/masks/masks.s

@@ -3,7 +3,7 @@
 #include "textflag.h"
 
 // func Masks() (uint16, uint64)
-TEXT ·Masks(SB), NOSPLIT, $0-16
+TEXT ·Masks(SB), NOSPLIT, $8-16
 	MOVQ $0x0001002a, AX
 	MOVQ $0x0002002a, CX
 	MOVQ $0x0003002a, DX

tests/alloc/upper32/upper32.s

@@ -3,7 +3,7 @@
 #include "textflag.h"
 
 // func Upper32() uint64
-TEXT ·Upper32(SB), NOSPLIT, $0-8
+TEXT ·Upper32(SB), NOSPLIT, $8-8
 	// Initialize sum.
 	XORQ AX, AX