ldscript.powerpc*: Keep .data.rel.ro out of PT_DYNAMIC

[mchoo]

Summary

When .got2 is empty, the orphan .data.rel.ro output section follows
.dynamic and inherits both its :kernel and :dynamic program-header
assignments. This makes PT_DYNAMIC cover .data.rel.ro as well as the
actual dynamic table. On powerpc64le, its resulting size may not even
be a multiple of Elf64_Dyn.

Describe .data.rel.ro explicitly and assign it only to :kernel,
terminating :dynamic inheritance on powerpc, powerpc64, and powerpc64le.

MFC after: 2 weeks
MFC to: stable/14, stable/15
Sponsored by: FreeBSD Foundation, Reliable Computer Systems Lab

Test Plan

readelf -l shows that .data.rel.ro does not appear in 01 (Dynamic) segment after this patch.
ppc64{be,le} boots. Not tested on ppc32.

[adrian]

Would you mind adding a comment explaining this above the section name so we don't forget why ?

[mchoo]

Reflect @adrian's suggestion

[adrian]

also just to be clear - did you boot test this on something? :-P :-)

[mchoo]

In D59620#1368197, @adrian wrote:

also just to be clear - did you boot test this on something? :-P :-)

Tested on ppc64{be,le} on IBM LC Power 9. I didn't test on ppc32 although I'm pretty sure it will pass.

[adrian]

I'm ok with it, let's wait for @jhibbits to chime in too.

[mchoo]

ping @jhb for mentor approval

edit: Do I need another mentor approval for MFC?