[PowerPC] Initialize ELFv1 as a secondary sysvec to ELFv2.

In order to enable VDSO timekeeping, it is necessary that there be exactly
one primary FreeBSD sysvec for each of the host and (optionally) compat32.

So, switch ELFv1 to being a secondary sysvec of ELFv2, so it does not get
double-allocated in the shared page.

Since secondary sysvecs use the same sigcode allocation as the primary,
define both to use the main sigcode64, and adjust the sv_sigcode_base on
ELFv2 after initialization to point to the correct offset.

This has the desirable side effect of avoiding having a separate copy of
the signal trampoline in the shared page. Our sigcode64 was already written
to take advantage of trampoline sharing, it was just not being allocated
that way until now.

Submitted by: jhibbits
Sponsored by: Tag1 Consulting, Inc.