Formalize the use of linker scripts for kernel modules.

Automatically apply ldscript.kmod.${MACHINE_ARCH} if it exists.
We already have an i386-specific linker script; rename it accordingly.

Note that the linker script is applied when the object files are
partially linked. (For amd64 this is also the final link.)

Reviewed by: imp, kib
Discussed with: jhb
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21887