RISC-V: Round up pre-populated stack frame to arch stack alignment

The stack frame size, used for context switch, is rounded up to 16-bytes
alignment. Therefore, we need round down the pointer of top of the
pre-populated stack frame so that the preserved stack frame size is also
rounded up to 16-bytes alignment.

Fixes #29535

Signed-off-by: Shih-Wei Teng <swteng@andestech.com>