Add some support for cross-compilation to MC68000 AmigaOS (#412)

Add m68k to recognized CPUs (machinetype)
Add amigaos to recognized operating systems (osversion)
Add ability to override "config.guess" output w/ LDEARCH= to osversion
Add platform.h clauses to recognize AmigaOS 3 using gcc to cross-compile

bin/machinetype

@@ -12,6 +12,7 @@
 os=${LDEARCH:-`./config.guess`}
 # o/s switch block
 case "$os" in
+        m68k-*)		      echo m68k ;;
         sparc-*)	      echo sparc ;;
         alpha-*)	      echo alpha ;;
 	i*86-*-*)             echo 386 ;;

bin/osversion

@@ -1,6 +1,7 @@
 #!/bin/sh
-os=`./config.guess`
+os=${LDEARCH:-`./config.guess`}
 case "$os" in
+        m68k-*-amigaos)		echo amigaos ;;
         sparc-sun-sunos*)	echo sunos4 ;;
         sparc-sun-solaris1*)	echo sunos4 ;;
         *-*-solaris2*)	        echo sunos5 ;;