Round TF_SIZE up to the stack alignment (16-bytes).

The kernel adjusts the stack by TF_SIZE and the RISC-V ABI requires
that it remain 16-byte aligned.

Reported by: CHERI, jrtc27
Reviewed by: mhorne
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26328