Fonts: Replace discarded const qualifier

Commit 6735b463 ("Fonts: Support FONT_EXTRA_WORDS macros for built-in
fonts") introduced the following error when building rpc_defconfig (only
this build appears to be affected):

 `acorndata_8x8' referenced in section `.text' of arch/arm/boot/compressed/ll_char_wr.o:
    defined in discarded section `.data' of arch/arm/boot/compressed/font.o
 `acorndata_8x8' referenced in section `.data.rel.ro' of arch/arm/boot/compressed/font.o:
    defined in discarded section `.data' of arch/arm/boot/compressed/font.o
 make[3]: *** [/scratch/linux/arch/arm/boot/compressed/Makefile:191: arch/arm/boot/compressed/vmlinux] Error 1
 make[2]: *** [/scratch/linux/arch/arm/boot/Makefile:61: arch/arm/boot/compressed/vmlinux] Error 2
 make[1]: *** [/scratch/linux/arch/arm/Makefile:317: zImage] Error 2

The .data section is discarded at link time.  Reinstating acorndata_8x8 as
const ensures it is still available after linking.  Do the same for the
other 12 built-in fonts as well, for ...