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

Reset errno in :scalbnf_val and :scalbnl_val before running the tests so the
tested errno isn't stale

This was needed in order for the test to pass on amd64 with stable/10

MFC after: 3 days

Reset errno to 0 before running scalbn to be sure that the tested errno is
valid

Sponsored by: EMC / Isilon Storage Division