Add get_FAC1_via_INDEX

Also change to only use bit 7 as sign in FAC1 and FAC2 (as the original BASIC does) to simplify
implementations.

src/basic/math_mov/ba90.get_FAC2_via_INDEX.s

@@ -33,20 +33,10 @@ get_FAC2_via_INDEX:
 
 	dey
 	lda (INDEX), y
+	sta FAC2_sign
 	ora #$80                           ; clear the sign bit
 	sta FAC2_mantissa+0
 
-	; Copy the sign
-
-	lda (INDEX), y
-	bpl @1                             ; assumption: non-negative numbers are more frequent
-	lda #$FF
-	+skip_2_bytes_trash_nvz
-@1:
-	lda #$00
-
-	sta FAC2_sign
-
 	; Copy the exponent
 
 	dey