[llvm-objcopy] Don't change permissions of non-regular output files

There is currently an EPERM error when a regular user executes `llvm-objcopy a.o /dev/null`.
Worse, root can even change the mode bits of /dev/null.

Fix it by checking if the output file is special.

A new overload of llvm::sys::fs::setPermissions with FD as the parameter
is added. Users should provide `perm & ~umask` as the parameter if they
intend to respect umask.

The existing overload of llvm::sys::fs::setPermissions may be deleted if
we can find an implementation of fchmod() on Windows. fchmod() is
usually better than chmod() because it saves syscalls and can avoid race
condition.

Reviewed By: jakehehrlich, jhenderson

Differential Revision: https://reviews.llvm.org/D64236

llvm-svn: 365753