Refer to stable wordref by direct offset in "hot zone" immediates

Previously, it was impossible to cross-compile Collapse OS from a
binary-offsetted Collapse OS because stable ABI wordrefs would have
a wrongly offsetted address.

This solves the problem by replacing those wordrefs by direct,
hardcoded stable ABI offset references.

blk/070

@@ -3,4 +3,4 @@ Implementation notes
 71 Execution model             73 Executing a word
 75 Stack management            77 Dictionary
 80 System variables            85 Word types
-89 Initialization sequence
+89 Initialization sequence     91 Stable ABI

blk/091

@@ -0,0 +1,16 @@
+Stable ABI
+
+Across all architectures, some offset are referred to by off-
+sets that don't change (well, not without some binary manipu-
+lation). Here's the complete list of these references:
+
+04 BOOT addr         06 (uflw) addr      08 LATEST
+42 EXIT wordref      53 (br) wordref     67 (?br) wordref
+80 (loop) wordref    a9 2>R wordref      bf (n) wordref
+d4 (s) wordref
+
+BOOT and (uflw) exist because they are referred to before those
+words are defined (in core words). LATEST is a critical part
+of the initialization sequence.
+
+                                                        (cont.)

blk/092

@@ -0,0 +1,16 @@
+Stable wordrefs are there for more complicated reasons. When
+cross-compiling Collapse OS, we use immediate words from the
+host and some of them compile wordrefs (IF compiles (?br),
+LOOP compiles (loop), etc.). These compiled wordref need to
+be stable across binaries, so they're part of the stable ABI.
+
+Another layer of complexity is the fact that some binaries
+don't begin at offset 0. In that case, the stable ABI doesn't
+begin at 0 either. The EXECUTE word has a special handling of
+those case where any wordref < 0x100 has the binary offset
+applied to it.
+
+But that's not the end of our problems. If an offsetted binary
+cross compiles a binary with a different offset, stable ABI
+references will be > 0x100 and be broken.
+                                                        (cont.)

blk/093

@@ -0,0 +1,3 @@
+For this reason, any stable wordref compiled in the "hot zone"
+(B397-B400) has to be compiled by direct offset reference to
+avoid having any binary offset applied to it.

blk/397

@@ -1,9 +1,9 @@
-( Now we have "as late as possible" stuff )
+( Now we have "as late as possible" stuff. See B70 and B270. )
 : _bchk DUP 0x7f + 0xff > IF LIT< br-ovfl (print) ABORT THEN ;
-: DO COMPILE 2>R H@ ; IMMEDIATE
+: DO 0xa9 ( 2>R ) , H@ ; IMMEDIATE
-: LOOP COMPILE (loop) H@ - _bchk , ; IMMEDIATE
+: LOOP 0x80 ( loop ) , H@ - _bchk , ; IMMEDIATE
 ( LEAVE is implemented in low xcomp )
-: LITN COMPILE (n) , ;
+: LITN 0xbf ( n ) , , ;
 ( gets its name at the very end. can't comment afterwards )
 : _ BEGIN LIT< ) WORD S= UNTIL ; IMMEDIATE
 : _ ( : will get its name almost at the very end )

blk/398

@@ -1,11 +1,11 @@
 : IF ( -- a | a: br cell addr )
-    COMPILE (?br) H@ 2 ALLOT ( br cell allot )
+    0x67 ( ?br ) , H@ 2 ALLOT ( br cell allot )
 ; IMMEDIATE
 : THEN ( a -- | a: br cell addr )
     DUP H@ -^ _bchk SWAP ( a-H a ) !
 ; IMMEDIATE
 : ELSE ( a1 -- a2 | a1: IF cell a2: ELSE cell )
-    COMPILE (br)
+    0x53 ( br ) ,
     2 ALLOT
     [COMPILE] THEN
     H@ 2- ( push a. -2 for allot offset )

blk/399

@@ -1,8 +1,9 @@
-: LIT< COMPILE (s) WORD DUP C@ 1+ MOVE, ; IMMEDIATE
+: LIT< 0xd4 ( s ) , WORD DUP C@ 1+ MOVE, ; IMMEDIATE
 : BEGIN H@ ; IMMEDIATE
-: AGAIN COMPILE (br) H@ - _bchk , ; IMMEDIATE
+: AGAIN 0x53 ( br ) , H@ - _bchk , ; IMMEDIATE
-: UNTIL COMPILE (?br) H@ - _bchk , ; IMMEDIATE
+: UNTIL 0x67 ( ?br ) , H@ - _bchk , ; IMMEDIATE
 : [ INTERPRET ; IMMEDIATE
 : ] R> DROP ;
 : COMPILE ' LITN ['] , , ; IMMEDIATE
 : [COMPILE] ' , ; IMMEDIATE
+: ['] ' LITN ; IMMEDIATE

blk/400

@@ -2,10 +2,9 @@
   is not an IMMEDIATE yet and will not be treated properly by
   xcomp. )
 : _
-    ['] EXIT ,
+    0x42 ( EXIT ) ,
     R> DROP     ( exit : )
 ; IMMEDIATE
-: ['] ' LITN ; IMMEDIATE
 ';' X' _ 4 - C! ( give ; its name )
 ':' X' _ 4 - C! ( give : its name )
 '(' X' _ 4 - C!