riscv: Align the trap handler to 64 bytes

This is required on CPUs which always operate in CLIC mode, such as the
T-HEAD E906 and E907. Per the CLIC specification: "In this mode, the
trap vector base address held in mtvec is constrained to be aligned on a
64-byte or larger power-of-two boundary."

Reported-by: Madushan Nishantha <jlmadushan@gmail.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>