fix -Werror regressions with gcc 4.8 on x86_64

[Roland McGrath]

I think what we want is to support building libc with NDEBUG but ensure
that this doesn't break the test suite.  One approach is some makefile
machinations to ensure that -DNDEBUG gets removed from the options when
compiling the test sources.  But I think the better approach is to change
all the test code using assert to use a different macro that exists just
for test code, and never elides the checks.