Fix definition of SSTATUS_SD

The SD bit is defined as the MSB of the sstatus register, meaning its
position will vary depending on the CSR's length. Previously, there were
two (unused) defines for this, for the 32 and 64-bit cases, but their
definitions were swapped.

Consolidate these into one define: SSTATUS_SD, and make the definition
dependent on the value of __riscv_xlen.

Reviewed by: br
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D23402