Add abs_FAC1

src/basic/math/bc58.abs_FAC1.s

@@ -0,0 +1,17 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
+; Math package - absolute value of FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named ABS.
+;
+
+abs_FAC1:
+
+    lsr FAC1_sign
+    rts

src/basic/math/bc58.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