runtime: copy runtime package time code from Go 1.7

runtime: copy runtime package time code from Go 1.7

    Fix handling of function values for -fgo-c-header to generate FuncVal*,
    not simply FuncVal.

    While we're here change runtime.nanotime to use clock_gettime with
    CLOCK_MONOTONIC, rather than gettimeofday.  This is what the gc library
    does.  It provides nanosecond precision and a monotonic clock.

    Reviewed-on: https://go-review.googlesource.com/31232

From-SVN: r241197