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/

libm: remove unused variables for LDBL_MANT_DIG != 113

Sponsored by: The FreeBSD Foundation

* msun/man/cosh.3:
* msun/man/sinh.3:
* msun/man/tanh.3:
. Fix grammar.

* msun/src/e_coshl.c:
* msun/src/e_sinhl.c:
. Fix comment.

* msun/src/s_tanhl.c:
. Remove unused variables.
. Fix location/indentation of comments.
. Use comparison involving ints instead of long double.
. Re-order polynomial evaluation on ld128 for |x| < 0.25.
For now, retain the older order in an "#if 0 ... #else" block.
. Use int comparison to short-circuit the |x| < 1.5 condition.

Requested by: bde

* Makefile:
. Hook coshl, sinhl, and tanhl into libm.
. Create symbolic links for corresponding manpages.
. While here remove a nearby extraneous space.

* Symbol.map:
* src/math.h:
. Move coshl, sinhl, and tanhl to their proper locations.

* man/cosh.3:
* man/sinh.3:
* man/tanh.3:
. Update the manpages.

* src/e_cosh.c:
* src/e_sinh.c:
* src/s_tanh.c:
. Add weak reference for LBDL_MANT_DIG==53 targets.

* src/imprecise.c:
. Remove the coshl, sinhl, and tanhl kludge.

* src/e_coshl.c:
. ld80 and ld128 implementation of coshl().

* src/e_sinhl.c:
. ld80 and ld128 implementation of sinhl().

* src/s_tanhl.c:
. ld80 and ld128 implementation of tanhl().

Obtained from: bde (mostly), das and kargl