Add set_FAC1_mantissa

src/basic/math/af87.interop

@@ -0,0 +1,3 @@
+This is an important floating point math routine, documented in many places.
+
+XXX - try to find real-life example

src/basic/math/af87.set_FAC1_mantissa.s

@@ -0,0 +1,20 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - sets mantissa to 00yyxxaa
+;
+; See also:
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+
+set_FAC1_mantissa:
+    sty FAC1_mantissa+1
+    stx FAC1_mantissa+2
+    sta FAC1_mantissa+3
+    lda #0
+    sta FAC1_mantissa
+    rts