Zircon kernel libc header layer

Source: zircon/kernel/lib/libc/include and include-after
Fuchsia revision: f8d670ab4bbb71bfb4d569893ba12fbb8a83bacd

This first import contains the complete public header layer.  The libc
implementation sources are intentionally not imported yet because abort,
panic, heap, console output, and C++ ABI providers belong to later foundation
batches.  Existing LumeOS C implementations continue to provide the symbols
used by the current kernel.