SCons: restore Werror option in src/SConscript
SCons: restore Werror option in src/SConscript

Partial backout of cset 8b223e308b08.

Although it's great that there's currently no need
for Werror=false in the current tree, some of us
have uncommitted code that still needs this option.

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