linux: use CLOCK_MONOTONIC_COARSE if available

clock_gettime(CLOCK_MONOTONIC) is only serviced from the vDSO when the
__vdso_clock_gettime() wrapper is confident enough that the vDSO timestamp is
accurate.  When in doubt, it falls back to making a traditional system call.

That's a commendable approach but overkill for our purposes because we don't
need nanosecond precision. That's why this commit changes the default clock to
CLOCK_MONOTONIC_COARSE when the kernel >= 2.6.32.  This change eliminates the
clock_gettime() system call altogether, even on virtualized systems.
linux: check clock resolution first, wip