sysutils/docker-buildx: fix build on armv7

On armv7, syscall.Timespec and golang.org/x/sys/unix.Timespec do not
have the same members due to padding in syscall.Timespec. Work around
this difference by converting field-by-field.

Approved by: portmgr (build fix blanket)
See also: https://github.com/tonistiigi/fsutil/issues/266