RFR (S): 8211175: Remove temporary clock initialization duplication

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8211175/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8211175

This cleans up some code duplication between os::Linux and os::Posix. 
os::Posix now exports the necessary clock functions so the Linux code 
can use those.

Also cleaned up the clock_getres syscall as that is already provided via 
the direct clock_getres (no need for a syscall).

Finally, as code in os::Linux already assumed clock_gettime etc was 
always available at build-time (and run-time for that matter) this is 
now explicitly checked via:

+ #ifndef SUPPORTS_CLOCK_MONOTONIC
+ #error "Build platform doesn't support clock_gettime and related 
functionality"
+ #endif

Testing: tiers 1-3 (mach5)

Thanks,
David