Use LLVM Clang explicitly on FreeBSD

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Unlike GNU find, FreeBSD's find needs a path before an expression

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Support FreeBSD on pkg/system/utimes_*.go

Implement system.LUtimesNano and system.UtimesNano. The latter might be
removed in future because it's basically same as os.Chtimes. That's why
the test is mainly focusing LUtimesNano.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Merge pull request #4878 from kzys/freebsd-utimes

Support FreeBSD on pkg/system/utimes_*.go