Tidy the _set_tp function for RISC-V.

• Use a constant for the offset instead of a magic number.
• Use an addi instruction that writes to tp directly instead of a mv that writes the result of a compiler-generated addi.

Reviewed by: mhorne
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23521