Switch to C11 noreturn to work around a compiler bug in llvm.

Elliott said:

Works around a clang bug in the version of clang shipping with Android T
that causes a miscompile in xstdio_create() on x86-64, that oddly only
affects __attribute__((__noreturn__)) and not _Noreturn!

Bug: http://b/225806996
Test: manual inspection of generated code on x86-64
Change-Id: I12302d2300fc454f92352b43090745c04ed66fd0