sim: bfin: only regen linux-fixed-code.h in maintainer mode

sim: bfin: only regen linux-fixed-code.h in maintainer mode

Since many people don't have a Blackfin toolchain available, only try to
regenerate the header file when in maintainer mode.  This file rarely changes,
and when it does, we commit the generated output, so there's almost never a
need to run directly on an end system.