soc: riscv: riscv-privilege: add support for mtvec vectored mode

Enable support for mtvec vectored mode for privilege SOCs. This
is an optional feature and takes up addtional code space. It is
necessary to support this feature for privilege SOCs that only
support mtvec vectored mode.

Change was tested on qemu_riscv32 and qemu_riscv64 boards with
CONFIG_RISCV_MTVEC_VECTORED_MODE enabled.

Signed-off-by: William Patty <wpatty24@gmail.com>