dtrace_asm.S: properly enter/exit user access

In order to read or write userspace memory without generating an access
fault, we must first enable the SUM bit in the sstatus CSR.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38298

(cherry picked from commit 0e563bde48b1af6bd472e940f04a8d76315749ab)