Remove $FreeBSD$: one-line .c pattern

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

Fix unused functions in invtrig_test.c

I only tested the WARNS=6 change on AArch64 and AMD64, but this file has
unused functions for architectures with LDBL_PREC == 53.

While touching this file change the LDBL_PREC == 53 checks to i386 checks.
The long double tests should only be disabled for i386 (due to the rather
odd rounding mode that it uses) not all architectures where long double
is the same as double.

PR: 205449
Fixes: 87d65c747a43 ("lib/msun: Allow building tests with WARNS=6")
Reported by: Jenkins

Improve test messages for msun tests

Also print the mismatched values when numbers compare not equal.

Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D29091

Convert the msun tests to ATF

This provides better error messages that just an assertion failure and
also makes it easier to mark individual tests as XFAIL.
It was also helpful when coming up with D28786 and D28787.

Differential Revision: https://reviews.freebsd.org/D28798

Fix warnings in lib/msun/tests/... to help pave way for WARNS?= 6.

- Staticize variables.
- Use nitems liberally. Wherever nitems is used, use unsigned integers
- Remove unused variables (argc, argv, etc)

This fixes most issues -- some issues remain in logarithm_test though.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

Integrate the remaining tools/regression/lib/msun testcases into the
FreeBSD test suite under lib/msun/tests

MFC after: 3 weeks
X-MFC with: r292328
Sponsored by: EMC / Isilon Storage Division