Wrappers folly::chrono::clock_gettime and clock_gettime_ns

Wrappers folly::chrono::clock_gettime and clock_gettime_ns

Summary:
On Linux hosts, the fast path to get the current time in
nanoseconds without doing a syscall to the kernel is available via the
VDSO kernel-runtime interface.

In this diff, I:

1. Expose portability wrappers `folly::chrono::clock_gettime()` and
`folly::chrono::clock_gettime_ns()`
2. Implement a VDSO wrapper on Linux hosts to implement those without
a round-trip to the kernel

Depends On D3418054

Reviewed By: bmaurer

Differential Revision: D3418087

fbshipit-source-id: 3fb99f0dd946f19ba29d0d52a1038dad3556bafd