Split config.bfd to use riscv{32,64}, not riscv*
Split config.bfd to use riscv{32,64}, not riscv*

I'm trying to rebase our binutils port off of upstream's git repo, and
this is one of the things I ended up noticing.  I believe this is the
correct thing to do, since we're now only supporting riscv32/riscv64
as our tuples.

manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain