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/

libc/tests/stdlib/dynthr_mod/dynthr_mod.c: mark dummy as used

It receives the malloc() result, and we do not want the malloc() call
to be optimized out, which is allowed for hosted compiler. Use dummy
for actual write though.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week

libc tests: dynthr_mod: fix some WARNS issues

This is being addressed as part of a side-patch I'm working on that builds
all the things with WARNS=6, instead of relying on it being supplied in just
shallow parts of the build with higher-level Makefile.inc.

Provide a prototype for mod_main and annotate the thread function argument
as unused.

MFC after: 1 week

libc/tests: Add test case for jemalloc/libthr bug fixed in r343566

Submitted by: Andrew Gierth (original reproducer; kevans massaged for atf)
Reviewed by: kib
MFC after: 2 weeks
X-MFC-with: r343566 (or after)
Differential Revision: https://reviews.freebsd.org/D19027