Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r246117:
Rework the __vdso_* symbols attributes to only make the symbols weak,
but use normal references instead of weak. This makes the statically
linked binaries to use fast gettimeofday(2) by forcing the linker to
resolve references and providing the neccessary functions.

MFC r237434:
Use struct vdso_timehands data to implement fast gettimeofday(2) and
clock_gettime(2) functions if supported.

Use struct vdso_timehands data to implement fast gettimeofday(2) and
clock_gettime(2) functions if supported. The speedup seen in
microbenchmarks is in range 4x-7x depending on the hardware.

Only amd64 and i386 architectures are supported. Libc uses rdtsc and
kernel data to calculate current time, if enabled by kernel.

Hopefully, this code is going to migrate into vdso in some future.

Discussed with: bde
Reviewed by: jhb
Tested by: flo
MFC after: 1 month