lib/msun: Allow building tests with WARNS=6

The only change needed is to mark a few variables as static.

Update libm tests from NetBSD

I did this without a full vendor update since that would cause too many
conflicts. Since these files now almost match the NetBSD sources the
next git subtree merge should work just fine.

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D28797

Import proper fix for misc/49356 (/usr/include/atf-c/config.h) after atf-c/config.h
was removed from the build

Pointyhat to: me (again, for not running make delete-old after running test builds)