mach: Add CLOCK_MONOTONIC case in clock_gettime()

mach: Add CLOCK_MONOTONIC case in clock_gettime()

The Mach RPC host_get_uptime64() is implemented. It returns the elapsed time
value since bootup. See

https://git.savannah.gnu.org/cgit/hurd/gnumach.git/commit/?id=fc494bfe3fb6363e1077dc035eb119970d84a9d1

In this patch, the RPC is used to implement the monotonic clock for
mach.

* config.h.in: Add HAVE_HOST_GET_UPTIME64 config entry
* sysdeps/mach/clock_gettime.c: Add CLOCK_MONOTONIC case
* sysdeps/mach/configure: Check the existence of host_get_uptime64 RPC
* sysdeps/mach/configure.ac: Check the existence of host_get_uptime64 RPC

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