Ignore new compiler warnings

Clang 6.0.1 update introduced a number of new warnings, failing
werror build. Ignore the -Wvisibility, -Wincompatible-pointer-types,
-Wformat warnings for external/compiler-rt.

Test: m checkbuild
Change-Id: I56ac95dbea43012949786625952d9a395addae84
2 files changed