More C compatibility.

Add C11 static_assert to <assert.h>. Remove uses of __dead while we're there:
__noreturn already does the same thing on those functions.

Fix <uchar.h> so it works from C.

<stdalign.h> and <stdnoreturn.h> are provided by clang, so there's nothing
for us to do.

Bug: http://b/29178582
Change-Id: Iebc46223868729a26d1a61eb125b76cbcb83a22d