It was brought to my attention that we're missing vdso clock_gettime
support for some archs. Based on the vdso(7) man page and a check of
what archs already have support in musl, powerpc, powerpc64, and s390x
seem to be the only ones that need to be added. For powerpc64, I'm not
clear on if there are any calling convention issues (ELFv2 vs v1) that
might make it not work; the other two I would expect to work, but I
don't have setups to test them, so I'd appreciate feedback from anyone
who can test and who'd like to get these patches in on whether they
work.

(Theoretically x32 might also should have vdso support, but I'm not
sure what the kernel maps in x32 processes and whether the code there
is usable from an x32 process.)

Rich