kernel: Sync ldscript files wrt gnu.attributes and .note.GNU-stack

Bring the keeping of .gnu.attributes to all architectures. Also discard
.note.GNU-stack on all archtiectures. Plus delete obsolete comment that
was removed from i386 in 2010.

Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44069

ldscript.powerpc*: Only put .dynamic in PT_DYNAMIC

Currently there are a few output sections left as implicitly using
:kernel :dynamic before :kernel on its own is used again, which means
they end up in both the PT_LOAD and the PT_DYNAMIC segments, an unusual
situation which the new libelf-based kldxref initially treated as
invalid. Thus, hoist the :kernel to the very next section to ensure only
.dynamic is in PT_DYNAMIC, as is more normal.

Whilst here, sync ldscript.powerpc64le with ldscript.powerpc64 to pick
up various fixes that were presumably made between the start of the
powerpc64le port and it being committed and got missed.

Reviewed by: jhibbits, jhb
Differential Revision: https://reviews.freebsd.org/D43066

sys: Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

[PowerPC64LE] Set up powerpc.powerpc64le architecture

This is the initial set up for PowerPC64LE.

The current plan is for this arch to remain experimental for FreeBSD 13.

This started as a weekend learning project for me and kinda snowballed from
there.

(More to follow momentarily.)

Reviewed by: imp (earlier version), emaste
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26399