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