cache: use syscall for clonefile
cache: use syscall for clonefile

This is to avoid error like
dyld: Symbol not found: _clonefile
https://chromium-swarm.appspot.com/task?id=5a2f0f760297ff10
on macOS 10.11

Bug: 1296318, 1315077
Change-Id: I9e1a5134ed58e5584ad18cc0b2ab8ce46c6a4bf4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3577202
Reviewed-by: Fumitoshi Ukai <0e88e065dd7f7d7f11da28b068df91c65342563c@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go