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/

AArch64: Don't set flush-subnormals-to-zero flag on startup

This flag has been set on startup since 65618fdda0f272a823e6701966421bdca0efa301.
However, This causes some of the math-related tests to fail as they report
zero instead of a tiny number. This fixes at least
/usr/tests/lib/msun/ldexp_test and possibly others.
Additionally, setting this flag prevents printf() from printing subnormal
numbers in decimal form.
See also https://www.openwall.com/lists/musl/2021/02/26/1

PR: 253847
Reviewed By: mmel
Differential Revision: https://reviews.freebsd.org/D28938

Fix :hexadecimal_floating_point on i386

Don't exclude i386 from LDBL_MANT_DIG == 64; it works properly in
that case.

While here, replace strcmp + atf_tc_fail with ATF_CHECK_MSG for 2
reasons:
- Gather as many results as possible instead of failing early and
not testing the rest of the cases.
- Simplify logic when checking test inputs vs outputs and printing
test result.

Tested on: amd64, i386
MFC after: 1 week
Sponsored by: Dell EMC Isilon

Remove spurious newlines from atf_tc_fail calls

This changes the results from broken (incorrect) to failed (correct) on
i386

MFC after: 1 week
Sponsored by: Dell EMC Isilon

Add missing va_ends for corresponding va_starts to clean up variable arguments
initialized in _test_fmt(..)

MFC after: 3 days
Reported by: cppcheck
Sponsored by: EMC / Isilon Storage Division

Fix the Indian numbering system (hi_IN.ISCII-DEV) tests

Submitted by: ache
X-MFC with: r290494 (if that ever happens)
Sponsored by: EMC / Isilon Storage Division

Integrate tools/regression/lib/libc/stdio into the FreeBSD test suite
as lib/libc/tests/stdio

- Fix some whitespace
- Convert the testcases to ATF
- Convert "/dev/null" to _PATH_DEVNULL

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division