riscv: spike: Fix memory leak in the board init

Coverity caught a malloc() call that was never freed. This patch ensures
that we free the memory but also updates the allocation to use
g_strdup_printf() instead of malloc().

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>