crtbank: put the bank switching helpers in HOME

__sdcc_banked_call and __sdcc_banked_ret were assembled into GSFINAL, which
sits in the startup path: GSINIT ends exactly where GSFINAL begins, so the
processor falls into it rather than being sent there. It works today only
because this object comes after every C object on the link line, so the
LJMP to __sdcc_program_startup is laid down first and the helpers land
behind it. Reordering that line, or moving main() into another module,
would put the helper body at the fallthrough address instead, and the board
would not come up out of a build that reports nothing wrong.

SDCC's own crtbank.asm declares the area order and then puts both symbols
in HOME, so the file takes that name and that preamble as well.

GSFINAL now holds the three byte jump and nothing else. Of 401 symbols 17
change address, every one in the startup region, and between the reset
vector and 0x0094 not a byte differs, so no interrupt vector is disturbed.

Run on a SWTGW218AS: it came back after about 42 seconds reporting the new
build, with its stored configuration byte identical and every link at the
speed it had before.

Makefile

@@ -118,7 +118,7 @@ $(BUILDDIR)/%.rel: %.asm
 	${ASM} ${AFLAGS} -o $@ $<
 #	mv -f $(addprefix $(basename $^), .lst .rel .sym) .
 
-$(BUILDDIR)/rtlplayground.ihx: $(OBJS) $(BUILDDIR)/crtstart.rel $(BUILDDIR)/crc16.rel
+$(BUILDDIR)/rtlplayground.ihx: $(OBJS) $(BUILDDIR)/crtbank.rel $(BUILDDIR)/crc16.rel
 	$(CC) $(CC_FLAGS) -Wl-bHOME=0x00000 -Wl-bBANK1=0x14000 -Wl-bBANK2=0x24000 -Wl-r -o $@ $^
 
 $(BUILDDIR)/rtlplayground.img: $(BUILDDIR)/rtlplayground.ihx

crtbank.asm

@@ -0,0 +1,29 @@
+	.area HOME    (CODE)
+	.area GSINIT0 (CODE)
+	.area GSINIT1 (CODE)
+	.area GSINIT2 (CODE)
+	.area GSINIT3 (CODE)
+	.area GSINIT4 (CODE)
+	.area GSINIT5 (CODE)
+	.area GSINIT  (CODE)
+	.area GSFINAL (CODE)
+	.area CSEG    (CODE)
+
+
+	.area HOME    (CODE)
+
+__sdcc_banked_call::
+	push	_PSBANK
+	xch	a,r0
+	push	a
+	mov	a,r1
+	push	a
+	mov	a,r2
+	anl	a,#0x1f
+	mov	_PSBANK, a
+	xch	a, r0
+	ret
+
+__sdcc_banked_ret::
+	pop	_PSBANK
+	ret

crtstart.asm

@@ -1,17 +0,0 @@
-	.area GSFINAL (CODE)
-
-__sdcc_banked_call::
-	push	_PSBANK
-	xch	a,r0
-	push	a
-	mov	a,r1
-	push	a
-	mov	a,r2
-	anl	a,#0x1f
-	mov	_PSBANK, a
-	xch	a, r0
-	ret
-
-__sdcc_banked_ret::
-	pop	_PSBANK
-	ret