Add tests for all C11 headers.

Well, all except <threads.h> which we have no current plans to implement.

The only thing this turned up is that clang's <float.h> doesn't define
FLT_HAS_SUBNORM/DBL_HAS_SUBNORM/LDBL_HAS_SUBNORM, so for now we assert
that those are missing so that when we get a fixed clang, we notice.

Also the <tgmath.h> is utterly perfunctory, but seems like an appropriate
amount of effort to put in.

Bug: N/A
Test: ran tests
Change-Id: I7f7a7040b3271ceda625bee8a40a9d8cef52ee44