This is a preparation to check if our users find noticeable bugs in the
x86-64 target, before we can decide if we want to remove the i686 target
for good.
Since the build now happens in Build/$SERENITY_ARCH/ and not in Build/,
this updates check-symbols.sh to use the correct directory to check the
LibC symbols in. For some reason, the constant failures did not show
up as errors in CI.