Always add a .note.GNU-stack section if -r.

With this patch, lld creates a .note.GNU_stack and adds that to an
output file if it is creating a re-linkable object file (i.e. if -r
is given). If we don't do this, and if you use GNU linkers as a final
linker, they create an executable whose stack area is executable,
which is considered pretty bad these days.

Differential Revision: https://reviews.llvm.org/D51400

llvm-svn: 340902