changelog.txt: corrections

Add ticket numbers for changelog entries:

Related to commits:
- 467b899
- 5062013

changelog.txt

@@ -23,7 +23,8 @@ Version 1.20.0
 - sf.net #985: Default access to a member symbol is disabled by a duplicated symbol in implicit namespace (regression of behaviour when fixing #948)
 - gas64: improvement of message when internal errors arise in gas64 emitter (SARG)
 - gas64: handle priority for constructors/destructors (SARG)
-- gas64: optimization 16 modified retrieval of registers and improved to handle specific cases (SARG)
+- sf.net #990: gas64: optimization 16 modified retrieval of registers and improved to handle specific cases (SARG)
+- sf.net #989: gas64: "rsi" replaced with "r11" in some symbols (SARG)
 - gas64: NEW UBYTE/BYTE[count] not handled (SARG)
 - sf.net #988: (regression) re-allow overloaded non-indexed array arguments - Can't pass UDT member array to an overloaded sub