sparc64, vdso: update the CLOCK_MONOTONIC_COARSE clock

sparc64, vdso: update the CLOCK_MONOTONIC_COARSE clock

In the significant rewrite involved in porting the SPARC vDSO code to
v4.1, the CLOCK_MONOTONIC_COARSE clock moved from being computed from
the CLOCK_REALTIME variable in the vvar page to being tracked by its own
vvar (mirroring a similar change done for x86): this vvar is maintained
in the same way, but is derived only once per jiffy tick rather than
every time clock_gettime() is called, which is likely faster under
sufficiently insane clock_gettime() than the way we did it in v3.0.

Unfortunately, the code to maintain this variable in
arch/sparc/kernel/vsyscall_gtod.c was never implemented, so
clock_gettime(CLOCK_MONOTONIC_COARSE) always returns zero. This is a
little coarser than the user would probably like.

Easily fixed by updating the relevant vvar in the same way as is done on
x86.

Reported-by: Wim Coekaerts <wim.coekaerts@oracle.com>
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Tested-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Orabug: 22137842