riscv: kexec: Align the kexeced kernel entry

The current riscv boot protocol requires 2MB alignment for RV64
and 4MB alignment for RV32.

In KEXEC_FILE path, the elf_find_pbase() function should align
the kexeced kernel entry according to the requirement, otherwise
the kexeced kernel would silently BUG at the setup_vm().

Fixes: 8acea455

 ("RISC-V: Support for kexec_file on panic")
Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Link: https://lore.kernel.org/r/20230906095817.364390-1-songshuaishuai@tinylab.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>