riscv/purgatory: align riscv_kernel_entry

In the Linux kernel, the following vulnerability has been resolved:

riscv/purgatory: align riscv_kernel_entry

When alignment handling is delegated to the kernel, everything must be

word-aligned in purgatory, since the trap handler is then set to the

kexec one. Without the alignment, hitting the exception would

ultimately crash. On other occasions, the kernel's handler would take

care of exceptions.

This has been tested on a JH7110 SoC with oreboot and its SBI delegating

unaligned access exceptions and the kernel configured to handle them.