Fix setjmp for RISC-V:
o The correct value for _JB_SIGMASK is 27.
o The storage size for double-precision floating

point register is 8 bytes.

Submitted by: "James Clarke" <jrtc4@cam.ac.uk>
Reviewed by: markj@
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16344