Fix build with recent binutils

Recent binutils considered the .gnu.warning.symbol section as a fatal error when
run with --fatal-warnings which makes any users of "insecure" functions from
libc failing to build with recent binutils.

Introduce a new macro: LD_FATAL_WARNINGS=no to run ld(1) with
--no-fatal-warnings for the users of "insecure" functions

Differential Revision:	https://reviews.freebsd.org/D1320


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276128 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f