Add SGN FAC1

src/basic/math/bc39.interop

@@ -0,0 +1,4 @@
+This is an important floating point math routine, documented in many places.
+
+Example code utilizing this routine:
+- https://github.com/irmen/prog8/blob/master/compiler/res/prog8lib/c64flt.p8

src/basic/math/bc39.sgn_FAC1.s

@@ -0,0 +1,20 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - evaluate FAC1 sign, to FAC1
+;
+; Output:
+; - FAC1 - 0 for FAC1 equal 0, 1 for FAC1 > 0, -1 for FAC1 < 0
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 115
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+sgn_FAC1:
+	jsr sgn_FAC1_A
+	; Fallthrough to convert_A_to_FAC1