libsrc/atari/system_check.s: fix MEMLO check

Old check forced the program's load address to be one more than
MEMLO. But MEMLO points to the first byte of usable RAM, not the last
byte of unusable RAM.

Problem found by Flashjazzcat, see https://github.com/cc65/cc65/issues/2964.

libsrc/atari/system_check.s

@@ -136,10 +136,10 @@ sys_ok: jsr     sdcheck         ; check for SpartaDOS-X, and if found, whether i
 
 sd_ok:  .include "xlmemchk.inc" ; calculate lowest address we will use when we move the screen buffer down
 
-        lda     MEMLO
-        cmp     lowadr
-        lda     MEMLO+1
-        sbc     lowadr+1
+        ;lda     lowadr         ; already loaded in xlmemchk.inc
+        cmp     MEMLO
+        lda     lowadr+1
+        sbc     MEMLO+1
         bcc     memlo_ok
 
 ; load address was too low
@@ -184,11 +184,11 @@ syschk:
 
         bcc     mem_err         ; program doesn't fit into memory
 
-        lda     MEMLO
-        cmp     #<__STARTADDRESS__
-        lda     MEMLO+1
-        sbc     #>__STARTADDRESS__
-        bcc     memlo_ok
+        lda     #<__STARTADDRESS__
+        cmp     MEMLO
+        lda     #>__STARTADDRESS__
+        sbc     MEMLO+1
+        bcs     memlo_ok
 
 ; load address was too low
         print_string2 lmemerr_txt, lmemerr_txt_len