sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Move PLTs to the beginning of amd64 kernel modules.

As with .text, the aim is to ensure that executable sections are
segregated from the rest, to avoid creation of writeable and executable
mappings. Recent versions of LLVM emit a PLT in firmware modules.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26444

Add an ldscript for amd64 kernel modules.

Use it to pad the text and read-only data sections to a 4KB boundary.
This will be used to enforce strict memory protections for some
sections of loadable kernel modules.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21970