powerpc: Add reloc_offset() to font bitmap pointer used for bootx_printf()

Since Linux v6.7, booting using BootX on an Old World PowerMac produces
an early crash. Stan Johnson writes, "the symptoms are that the screen
goes blank and the backlight stays on, and the system freezes (Linux
doesn't boot)."

Further testing revealed that the failure can be avoided by disabling
CONFIG_BOOTX_TEXT. Bisection revealed that the regression was caused by
a change to the font bitmap pointer that's used when btext_init() begins
painting characters on the display, early in the boot process.

Christophe Leroy explains, "before kernel text is relocated to its final
location ... data is addressed with an offset which is added to the
Global Offset Table (GOT) entries at the start of bootx_init()
by function reloc_got2(). But the pointers that are located inside a
structure are not referenced in the GOT and are therefore not updated by
reloc_got2(). It is therefore needed to apply the offset manually by using
PTRRELOC() macro."

Cc: stable@vger.kernel.org
Link: https://lists.debian.org/debian-powerpc/2025/10/msg00111.html
Link: https://lore.kernel.org/linuxppc-dev/d81ddca8-c5ee-d583-d579-02b19ed95301@yahoo.com/
Reported-by: Cedar Maxwell <cedarmaxwell@mac.com>
Closes: https://lists.debian.org/debian-powerpc/2025/09/msg00031.html
Bisected-by: Stan Johnson <userm57@yahoo.com>
Tested-by: Stan Johnson <userm57@yahoo.com>
Fixes: 0ebc7fe ("powerpc: Use shared font data")
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/22b3b247425a052b079ab84da926706b3702c2c7.1762731022.git.fthain@linux-m68k.org

Author: fthain

arch/powerpc/kernel/btext.c

@@ -20,6 +20,7 @@
 #include <asm/io.h>
 #include <asm/processor.h>
 #include <asm/udbg.h>
+#include <asm/setup.h>
 
 #define NO_SCROLL
 
@@ -463,7 +464,7 @@ static noinline void draw_byte(unsigned char c, long locX, long locY)
 {
 	unsigned char *base	= calc_base(locX << 3, locY << 4);
 	unsigned int font_index = c * 16;
-	const unsigned char *font	= font_sun_8x16.data + font_index;
+	const unsigned char *font = PTRRELOC(font_sun_8x16.data) + font_index;
 	int rb			= dispDeviceRowBytes;
 
 	rmci_maybe_on();