Fix Clang -Wunreachable-code-aggressive warnings

Fix Clang -Wunreachable-code-aggressive warnings

Based on the change in libaom:
fe36011455 Fix Clang -Wunreachable-code-aggressive warnings

Clang's -Wunreachable-code-aggressive flag enables several warning flags
such as -Wunreachable-code-break and -Wunreachable-code-return. Chrome's
build system enables -Wunreachable-code-aggressive (in
build/config/compiler/BUILD.gn), so it would be good if libvpx could be
compiled without -Wunreachable-code-aggressive warnings.

This requires the VPX_NO_RETURN macro be defined correctly for all the
compilers we support, otherwise some compilers may warn about missing
return statements after a die() or fatal() call (which does not return).

Change-Id: I0c069133af45a7a61759538b6d74c681ea087dcd