lib/msun: Cleanup after $FreeBSD$ removal

Remove no longer needed explicit inclusion of sys/cdefs.h.

PR: 276669
MFC after: 1 week

Remove $FreeBSD$: one-line .c pattern

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

Clean up libm use of the __ieee754_ prefix

This removes the __ieee754_ prefix from a number of the math functions.
msun/src/math_private.h contains the statement that

/*
* ieee style elementary functions
*
* We rename functions here to improve other sources' diffability
* against fdlibm.
*/
#define __ieee754_sqrt sqrt
...

Here, fdlibm refers to https://netlib.org/fdlibm. It is seen from
https://netlib.org/fdlibm/readme that this prefix was used to
differentiate between different standards:

Wrapper functions will twist the result of the ieee754
function to comply to the standard specified by the value
of _LIB_VERSION
if _LIB_VERSION = _IEEE_, return the ieee754 result;
if _LIB_VERSION = _SVID_, return SVID result;
if _LIB_VERSION = _XOPEN_, return XOPEN result;
if _LIB_VERSION = _POSIX_, return POSIX/ANSI result.
(These are macros, see fdlibm.h for their definition.)

AFAICT, FreeBSD has never supported these wrappers. In addition, as C99,
principally the long double, functions were added to libm, this
convention was not maintained. Given that only 148 of 324 files under
lib/msun contain a "Copyright (C) 1993 by Sun Microsystems" statement,
the removal of the __ieee754_ prefix provides consistency across all
source files.

The last time someone compared lib/msun to fdlibm appears to be

commit 3f70824172feb82ea3dcdb3866b54fe0eb7cd890
Author: David Schultz <das@FreeBSD.org>
Date: Fri Feb 4 18:26:06 2005 +0000

Reduce diffs against vendor source (Sun fdlibm 5.3).

The most recent fdlibm RCS string that appears in a Sun Microsystem
copyrighted file is date "95/01/18". With Oracle Corporation's
acquisition of Sun Microsystems in 2009, it is unlikely that fdlibm will
ever be updated. A search for fdlibm at https://opensource.oracle.com/
yields no hits.

Finally, OpenBSD removed the use of this prefix over 21 years ago. pSee
revision 1.6 of OpenBSD's math_private.h.

Note: this does not drop the __ieee754_ prefix from the trigonometric
argument reduction functions, e.g., __ieee754_rem_pio2. These functions
are internal to the libm and exported through Symbol.map; and thus,
reserved for the implementation.

PR: 272783
MFC after: 1 week

Fix some regressions caused by the switch from gcc to clang. The fixes
are workarounds for various symptoms of the problem described in clang
bugs 3929, 8100, 8241, 10409, and 12958.

The regression tests did their job: they failed, someone brought it
up on the mailing lists, and then the issue got ignored for 6 months.
Oops. There may still be some regressions for functions we don't have
test coverage for yet.

Various changes to improve the accuracy and speed of log{2,10}{,f}.
- Rename __kernel_log() to k_log1p().
- Move some of the work that was previously done in the kernel log into
the callers. This enables further refactoring to improve accuracy or
speed, although I don't recall the details.
- Use extra precision when adding the final scaling term, which improves
accuracy.
- Describe and work around compiler problems that break some of the
multiprecision calculations.

A fix for a small bug is also included:
- Add a special case for log*(1). This is needed to ensure that log*(1) == +0
instead of -0, even when the rounding mode is FE_DOWNWARD.

Submitted by: bde

Style fixes and updates to comments.

Submitted by: bde

Another minor nit: Make sure the constant here is a float so the compiler
doesn't promote the entire expression to double.

Fix various nits in style and comments that were pointed out by bde.
Code changes verified with md5.

Add log2() and log2f().