Disable warnings triggered in Clang r271374

http://b/28149048
http://b/29823425

Disable -Wvarargs that gets triggered in
system/core/debuggerd/test/ptrace_fake.cpp:ptrace_fake.  The second bug
above tracks that this warning gets re-enabled.

Test: Tested build, boot and common usage for Arm, Arm64, x86, x86_64,
Mips images in AOSP and internal branch.

Change-Id: Ife68675a77431bcc4717b4da297c14dc486d1413