libroot/posix: implement arch-specific parts for riscv64

Change-Id: Ia1eac32b383857724dd00b227a735174ff6f0a63
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3984
Reviewed-by: X512 <danger_mail@list.ru>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

libroot/riscv64: Add missing sigjmp's.

Change-Id: Id353f86571ddf5bc106af7c77f0bd3d22b5b7cbd
Reviewed-on: https://review.haiku-os.org/c/1381
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>