Add more linker protections around variables accessed from 16bit mode.

Rename VAR16 to VAR16_32 -- that macro supports accesses from both
    16bit and 32bit mode.
Introduce a new macro VAR16 that must be present on all global
    variables accessed from 16bit mode.
diff --git a/Makefile b/Makefile
index 24c67c2..3a24433 100644
--- a/Makefile
+++ b/Makefile
@@ -76,7 +76,7 @@
 
 $(OUT)%.proc.16.s: $(OUT)%.16.s
 	@echo "  Moving data sections to text in $@"
-	$(Q)sed 's/\t\.section\t\.rodata.*// ; s/\t\.data//' < $< > $@
+	$(Q)sed 's/^\t\.section\t\.\(ro\)\?data.*// ; s/^\t\.data$$//' < $< > $@
 
 $(OUT)%.16.s: %.c
 	@echo "  Compiling to assembler $@"