sparc64, vdso: Add gettimeofday() and clock_gettime().

sparc64, vdso: Add gettimeofday() and clock_gettime().

This commit adds gettimeofday() and clock_gettime() entry points to the SPARC64
vDSO: in conjunction with a suitably-modified glibc this provides a speedup to
gettimeofday(), time() and some clock_gettime() calls of on the order of
10--15x (the higher figure is for the coarse clock_gettime() clocks).

gettimeofday() and clock_gettime() use largely separate code paths: all
other approaches with less code duplication (e.g. doing all the work in
a struct timespec and only shifting it into a struct timeval before return)
turned out slower.

Tested on %stick-capable machines only: %tick codepaths untested.

Orabug: 20861959
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
(cherry picked from commit e8f9e5cfc9d297c7ebbc459b677ee0b8a3e45154)