riscv: Add initial SBI support

Add an SBI call function and immediately apply it to properly
exiting the test (instead of hanging) by invoking SBI shutdown
from exit(). Also seed an SBI test file wi

riscv: Add initial SBI support

Add an SBI call function and immediately apply it to properly
exiting the test (instead of hanging) by invoking SBI shutdown
from exit(). Also seed an SBI test file with a simple SBI test
that checks mvendorid is correctly extracted.

Run with e.g.
qemu-system-riscv64 -nographic -M virt \
-kernel riscv/sbi.flat \
-cpu rv64,mvendorid=45 \
-initrd sbi-env

and be happy that ctrl-a c q is no longer necessary to return to
the shell prompt. sbi-env has MVENDORID=45 in it.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
Acked-by: Thomas Huth <thuth@redhat.com>

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