libcxx: Remove -flto from LDFLAGS

libcxx: Remove -flto from LDFLAGS

It seems the buildbots can't handle it.

Added a cmake option to find the cxxabi files as they are part of the
toolchain and not in the normal path. It doesn't seem to make a
difference, just gets rid of cmake warnings.

Added another small GCC warning fix. It's fairly minor.

This has no change in compiled size, and most likely no change in
behavior. Bumped the PKG_RELEASE anyway.

Signed-off-by: Rosen Penev <rosenp@gmail.com>