riscv: enable mseal sysmap for RV64

Provide support for CONFIG_MSEAL_SYSTEM_MAPPINGS for RV64, covering the
vdso, vvar.

Passed sysmap_is_sealed and mseal_test self tests.
Passed booting a buildroot rootfs image and a cli debian rootfs image.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Cc: Jeff Xu <jeffxu@chromium.org>
Link: https://lore.kernel.org/r/20250426135954.5614-1-jszhang@kernel.org


Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>