Maintain a stack alignment of 16-bytes.

This violated the RISC-V 64-bit ABI. On CheriBSD this manifested as
a trap when attempting to store 16-byte aligned types while zeroing
the stack.

Reported by: CHERI, jrtc27
Reviewed by: mhorne, br
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26327