arm: Use "nr" constraint for Systemtap probes [BZ #24164]

With the default "nor" constraint, current GCC will use the "o"
constraint for constants, after emitting the constant to memory.  That
results in unparseable Systemtap probe notes such as "-4@.L1052".
Removing the "o" alternative and using "nr" instead avoids this.

(cherry picked from commit f1ac7455)