Add a switch macro

[jpelletier]

These macros specific for the AVR microcontrollers add a switch construct

;;;-----------------------------------------------------------------
;;; Switch
;;; _sw_case R16-31,Value
;;; _case R16-31,Value
;;; _break_sw
;;; _default
;;; _end_sw
;;;-----------------------------------------------------------------
.macro _sw_case REG VALUE
cpi \REG,\VALUE
.set __ST_IFNEST, __ST_IFNEST+2
_st_mkjmp _st_jmp_not_eq, %__ST_IFNEST, f
.endm

.macro _break_sw
_st_mkjmp _st_jmp_always, %__ST_IFNEST+1, f
.endm

.macro _case REG VALUE
_st_label %__ST_IFNEST
cpi \REG,\VALUE
_st_mkjmp _st_jmp_not_eq, %__ST_IFNEST, f
.endm

.macro _default
_st_label %__ST_IFNEST
.endm

.macro _end_sw
_st_label %__ST_IFNEST
_st_label %__ST_IFNEST+1
.set __ST_IFNEST, __ST_IFNEST-2
.endm