build: Add legacy support for futimens() and utimensat().

In order to allow using utimensat() in install(1), add futimens() and
utimensat() to -legacy.

The files futimens.c and utimensat.c are modified copies of the files under
lib/libc/sys/ since the libc versions use symbols that do not exist in the
libc on the build system (sys_futimens and sys_utimensat) . I expect the
next non-sweeping change to both sets of files to be to delete them, anyway.

This will allow reverting r299942 (which is a revert of r299850) enabling
nanosecond timestamps in install(1).

Reviewed by: bdrewery