For now these are just placeholders that execute the syscall.  We will
later optimize them to avoid kernel crossings, but we'd like to have the
VDSO entries from the first released kernel version to make the ABI
simpler.
Signed-off-by: NAndrew Waterman <andrew@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>