Would anyone have any objections to the idea of creating fs_filecore.h
so that it can be included by arch/arm/arm/disksubr.c ?

The non-acorn ARM ports don't need arch/arm/arm/disksubr_acorn.c, but
there isn't currently any way of testing at compile time whether the
calls to filecore_label_* are needed.

There is an opt_mbr.h file that can be used to conditionally compile
out the reference to the mbr_label_* calls.

Robert Swindells