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
diff --git a/debuggerd/Android.mk b/debuggerd/Android.mk
index 19c7298..fdedb76 100644
--- a/debuggerd/Android.mk
+++ b/debuggerd/Android.mk
@@ -130,6 +130,9 @@
     -Wno-missing-field-initializers \
     -fno-rtti \
 
+# Bug: http://b/29823425 Disable -Wvarargs for Clang update to r271374
+debuggerd_cpp_flags += -Wno-varargs
+
 # Only build the host tests on linux.
 ifeq ($(HOST_OS),linux)