Make dos errno multimapping linkage tinier

640 bytes for old kDos2Errno table
182 bytes for new kDos2Errno under hello2.com (MODE=fastbuild)
122 bytes for new kDos2Errno under hello2.com (MODE=tiny)

libc/sysv/gen.sh

@@ -29,9 +29,17 @@ scall() {
 }
 
 syscon() {
+  if [ -f "libc/sysv/dos2errno/${2/$/-}.S" ]; then
+    EXT=".S"
+  else
+    EXT=".s"
+  fi
   {
     echo ".include \"o/libc/sysv/consts/syscon.internal.inc\""
     echo ".syscon" "$*"
+    if [ -f "libc/sysv/dos2errno/${2/$/-}.S" ]; then
+      echo ".yoink kDos2Errno.${2/$/-}"
+    fi
   } >"$dir/${2/$/-}.s"
 }
 
@@ -59,3 +67,23 @@ errfun() {
 ' "$ERRNO" "$NAME"
   } >"$dir/${1/$/-}.S"
 }
+
+dos() {
+  DOS="$1"
+  UNIX="$2"
+  if ! [ -f "$dir/${UNIX/$/-}.S" ]; then
+    cat <<EOF >"$dir/${UNIX/$/-}.S"
+//	generated by libc/sysv/dos2errno.sh
+#include "libc/nt/errors.h"
+	.macro	.e doscode systemv
+	.short	\doscode
+	.long	\systemv
+	.endm
+	.section .sort.rodata.dos2errno.2,"a",@progbits
+	.globl	kDos2Errno.${UNIX}
+	.type	kDos2Errno.${UNIX},@object
+kDos2Errno.${UNIX}:
+EOF
+  fi
+  printf %s\\n "	.e	${DOS},${UNIX}" >>"$dir/${UNIX/$/-}.S"
+}