Zircon public support headers required by the first kernel library batch

Fuchsia revision: f8d670ab4bbb71bfb4d569893ba12fbb8a83bacd

Imported files:
  zircon/system/public/zircon/compiler.h
  sdk/lib/zircon-assert/zircon/assert.h

The assert header is a regular-file copy of the SDK target.  The corresponding
path under zircon/system/public is a Fuchsia-tree-relative symbolic link and
is deliberately not reproduced as a broken link.