linux: use CLOCK_MONOTONIC_COARSE if available

On some systems, clock_gettime(CLOCK_MONOTONIC) is only serviced from
the vDSO when the __vdso_clock_gettime() wrapper is confident enough
that the vDSO timestamp is highly accurate.  When in doubt, it falls
back to making a traditional SYS_clock_gettime system call with all
the overhead that entails.

While a commendable approach, it's overkill for our purposes because we
don't usually need high precision time. That's why this commit changes
the default clock to CLOCK_MONOTONIC_COARSE, provided it supports a
granularity of one millisecond or better.

This change should eliminate the system call on almost all systems,
including virtualized ones, provided the kernel is >= 2.6.32 and your
glibc is new enough to find and parse the vDSO.