syscall emulation: fix fast build issue
syscall emulation: fix fast build issue

Surprisingly gcc will complain about unused variables even
inside an 'if (false)' block.

I thought I had tested this previously, but apparently not.

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