Atari: fix interrupt handling if extended memory is banked in

Extendend memory is mapped over the main memory in the 0x4000..0x7FFF
area. Many DOSes disable interrupts while extended memory is banked in,
but not all (e.g. SpartaDOS-X).
This change modifies the initial interrupt handler to map in main memory
before chaining to the "worker" handlers.
Since the initial interrupt handler uses a data segment to store the
trampoline to chain to the original handler, introduce a new "LOWBSS"
segment to hold this trampoline. Otherwise the trampoline may end up
inside the 0x4000..0x7FFF area.

Add a link time warning if "LOWCODE" segment lays within the extended
memory window.

libsrc/atari/irq.s

@@ -16,6 +16,8 @@
 .segment        "ONCE"
 
 initirq:
+        lda     #$4C                    ; JMP opcode
+        sta     IRQInd
         lda     VVBLKD
         ldx     VVBLKD+1
         sta     IRQInd+1
@@ -45,17 +47,22 @@ IRQStub:
 .ifdef CHARGEN_RELOC
         lda     CHBAS
         pha
+.endif
 .endif
         lda     PORTB
         pha
-        and     #$FE
-        sta     PORTB                   ; disable ROM
+.ifdef __ATARIXL__
+        and     #$FE                    ; disable ROM
+.endif
+        ora     #$10                    ; map main memory into $4000..$7FFF area
+        sta     PORTB
+.ifdef __ATARIXL__
         set_chbase >__CHARGEN_START__
 .endif
         jsr     callirq                 ; Call the functions
-.ifdef __ATARIXL__
         pla
-        sta     PORTB                   ; restore old ROM setting
+        sta     PORTB                   ; restore old memory settings
+.ifdef __ATARIXL__
 .ifdef CHARGEN_RELOC
         pla
         sta     CHBAS
@@ -66,6 +73,8 @@ IRQStub:
 
 ; ------------------------------------------------------------------------
 
-.data
+.segment        "LOWBSS"
 
-IRQInd: jmp     $0000
+IRQInd: .res    3
+
+.end