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/

Merge in the latest SoftFloat changes from NetBSD. (NetBSD isn't the
original vendor, but we're using their heavily modified version.)
This brings in functions for long double emulation (both extended and
quad formats), which may be useful for testing, and also for replacing
libc/sparc64/fpu/.