riscv: Implement non-stub vdso_gettc and vdso_gettimekeep
PR: 256905
Reviewed by: arichardson, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30963
(cherry picked from commit 348c41d1815dc2e872a1deba1f4bf760caaa1094)