Fixes bug causing ReleaseSmall to fail on Windows.

Due to the change in default behavior of ReleaseSmall, debug info are
stripped by default. However because `Compile.create` still defaults to
null, `producesPdbFile` will report true for
`lib/std/Build/Step/InstallArtifact.zig` causing it to fail on copying a
file that does not exist. This commit change the default of strip
depending on `optimize`.