overwrite segment tests

asm/listing linker CFG support
asm and asm/listing test documentation

test/asm/listing/200-overwrite.s

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+; verification of overwrite segment feature
+; See: https://github.com/cc65/cc65/issues/1366
+
+; A: full memory area which is overwritten to the end
+.segment "A"
+.byte 0,1,2,3,4,5,6,7
+.segment "AO"
+.byte $24,$25,$26,$27
+
+; B: incomplete memory area overwritten in the fill area
+.segment "B"
+.byte 0,1,2
+.segment "BO"
+.byte $25,$26
+
+; C: memory area with gap overwritten across the gap
+.segment "C0"
+.byte 0,1,2
+.segment "C1"
+.byte 5,6,7
+.segment "CO"
+.byte $22,$23,$24,$25
+
+; D: incomplete memory area without fill,
+;    but overwrite extends past existing segments
+.segment "D"
+.byte 0,1,2
+.segment "DO"
+.byte $24,$25