[PECOFF] Enable dead-stripping even if /debug option is given.

/debug makes MSVC link.exe to not remove unused sections from
the resulting executable. We did the same thing before. However,
I realized that the removal of associative section depends on
the dead-stripping pass in LLD, so we cannot disable that. Or
LLD may produce slightly broken executables that have too much
data in it (which could result in nasty subtle bugs).

This patch is a temporary measure to create correct executable.
Currently /debug does not have any real effect for LLD anyway.

I'll improve associative section handling in another patch, so that
they are removed from output without depending on the dead-stripping
pass.