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.

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin