libc: fix a few misc warnings

Summary:
Fixes the following:
  o literal-suffix in libc/include/sys/cdefs_elf.h
  o pointer-sign in libc/upstream-freebsd/lib/libc/stdio/mktemp.c
  o implicit-function-declaration in libc/upstream-netbsd/libc/isc/ev_timers.c

Test Plan: verify

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D567