let clang driver pass through -O level

This makes compiling for size (-Os and -Oz) possible. -std-link-opts is still
added, but only for higher -O levels, ie. -O2 and -O3.
only add std-link-opts with -O3
perform all optimizations at link-time

Pass -disable-llvm-optzns to clang so that the first opt (during .bc compiling)
does not optimize at all.
Merge pull request #2 from alexjordan/opt-levels

patmos-clang defaults to -O0, bitcode optimizations done at link time