Fix SYS_arch_prctl handling regression
Fix SYS_arch_prctl handling regression

Fixes a regression introduced in 473a1c29d7994672076a6513990cffb51a2afa86
in handling SYS_arch_prctl.

Review-URL: https://codereview.appspot.com/213370043

sigma-random/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,code4bones/dynamorio,sigma-random/dynamorio,AmesianX/dynamorio