syscall_emul: fix bug in aux vector initialization
syscall_emul: fix bug in aux vector initialization

Writing 16 bytes from an 8-byte source value is a bad idea.
This doesn't appear to have broken anything, but showed up
as spurious differences when tracediffing runs.

rjschof/gem5,cancro7/gem5,aclifton/cpeg853-gem5,qizenguf/MLC-STT,powerjg/gem5-ci-test,sobercoder/gem5,gedare/gem5,powerjg/gem5-ci-test,TUD-OS/gem5-dtu,HwisooSo/gemV-update,gem5/gem5,TUD-OS/gem5-dtu,cancro7/gem5,gedare/gem5,SanchayanMaity/gem5,rallylee/gem5,TUD-OS/gem5-dtu,Weil0ng/gem5,gem5/gem5,Weil0ng/gem5,aclifton/cpeg853-gem5,gedare/gem5,briancoutinho0905/2dsampling,SanchayanMaity/gem5,powerjg/gem5-ci-test,gem5/gem5,rjschof/gem5,rallylee/gem5,rjschof/gem5,HwisooSo/gemV-update,sobercoder/gem5,gedare/gem5,sobercoder/gem5,gedare/gem5,sobercoder/gem5,cancro7/gem5,sobercoder/gem5,HwisooSo/gemV-update,Weil0ng/gem5,rallylee/gem5,qizenguf/MLC-STT,aclifton/cpeg853-gem5,KuroeKurose/gem5,Weil0ng/gem5,KuroeKurose/gem5,KuroeKurose/gem5,cancro7/gem5,qizenguf/MLC-STT,gedare/gem5,gem5/gem5,rjschof/gem5,SanchayanMaity/gem5,briancoutinho0905/2dsampling,SanchayanMaity/gem5,cancro7/gem5,rjschof/gem5,rjschof/gem5,rallylee/gem5,SanchayanMaity/gem5,aclifton/cpeg853-gem5,powerjg/gem5-ci-test,powerjg/gem5-ci-test,gem5/gem5,Weil0ng/gem5,aclifton/cpeg853-gem5,briancoutinho0905/2dsampling,SanchayanMaity/gem5,sobercoder/gem5,Weil0ng/gem5,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,briancoutinho0905/2dsampling,gem5/gem5,briancoutinho0905/2dsampling,KuroeKurose/gem5,rjschof/gem5,rallylee/gem5,powerjg/gem5-ci-test,Weil0ng/gem5,rallylee/gem5,TUD-OS/gem5-dtu,gem5/gem5,cancro7/gem5,KuroeKurose/gem5,powerjg/gem5-ci-test,aclifton/cpeg853-gem5,SanchayanMaity/gem5,KuroeKurose/gem5,cancro7/gem5,TUD-OS/gem5-dtu,HwisooSo/gemV-update,HwisooSo/gemV-update,HwisooSo/gemV-update,qizenguf/MLC-STT,sobercoder/gem5,briancoutinho0905/2dsampling,briancoutinho0905/2dsampling,gedare/gem5,qizenguf/MLC-STT,KuroeKurose/gem5,HwisooSo/gemV-update,qizenguf/MLC-STT,rallylee/gem5,qizenguf/MLC-STT,TUD-OS/gem5-dtu