libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

[PPC64] strncpy optimization

Assembly optimization of strncpy for PowerPC64, using double words
instead of bytes to copy strings.

Submitted by: Leonardo Bianconi <leonardo.bianconi_eldorado.org.br> (original version)
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D15369