Revive Amiga port for NetHack 3.7

Update the Amiga Intuition window port (AMII/AMIV) for the 3.7
window_procs API. Key changes:

- Update all window function signatures for 3.7
- Add assembly trampolines for AmigaOS register-based callbacks
- Convert all K&R function definitions to C99
- Add cross-compilation build system (cross-pre1/pre2/post.370)
  using bebbo's m68k-amigaos-gcc with -noixemul -std=gnu17 -m68000
- Clipping fixes: viewport centering, simplified ScrollRaster,
  duplicate Ctrl-R suppression, glyph buffer invalidation
- Add menucolor support in menu rendering
- Move native txt2iff.c and xpm2iff.c to outdated/
- Add nethack.cnf and README.amiga

sys/unix/hints/include/cross-pre1.370

@@ -38,6 +38,16 @@ override TARGETPFX = $(TARGETDIR)/
 override TARGET_LIBS=
 endif
 
+ifdef CROSS_TO_AMIGA
+CROSS=1
+BUILD_TARGET_LUA=1
+CROSS_SHARED=1
+override TARGET = amiga
+override TARGETDIR=../targets/$(TARGET)
+override TARGETPFX = $(TARGETDIR)/
+override TARGET_LIBS=
+endif
+
 ifdef CROSS
 override PREGAME=
 override BUILDMORE=