Since ASLR got enabled for ELFv2, the random bases for the heap, the
dynamic linker and PIE binaries can land anywhere in the 52-bit address
space radix gives us. A lot of userland doesn't like that: SpiderMonkey,
JavaScriptCore, QV4 and friends pack pointers into 47 or 48 bits and
crash on startup (gjs, cjs, qml, ...).
Do what amd64 does on LA57 hardware: keep the full range in the kernel
and give processes a 128 TiB map by default through a second set of
sysvecs. The 52 bits stay available per process via the la57 feature
control note (elfctl(1)) and PROC_LA_CTL (proccontrol(1)), and
vm.pmap.prefer_uva_la48 flips the default, all with the same names and
values as on amd64. The la48/la57 names don't describe POWER paging
modes, they come from that interface.
Fixes: 30ed27ff2556 ("sys/powerpc/powerpc/elf64_machdep.c: enable ASLR on ELFv2")
MFC after: 3 days