config/scripts/switch-arch: fix build on aarch64 and riscv64 hosts

Ensure that the host compiler builds with -DNATIVE_BITWIDTH_SMALLER_THAN_HOST_BITWIDTH
on aarch64 and riscv64 hosts for 32-bit target architectures, otherwise host pointers
can end up being truncated to 32-bits causing a segfault running forthstrap.

Resolves: https://github.com/openbios/openbios/issues/18
Resolves: https://github.com/openbios/openbios/issues/25
Closes: https://github.com/openbios/openbios/pull/12
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>