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.UtimesNano and system.LUtimesNano.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)