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