This change:

* splits //zircon/system/ulib/zxcrypt into two libraries, so that the
  FDIO dependencies don't leak into the driver, and vice versa
* reduces the set of dependencies of the driver to those actually
  required
* cuts out some transitive dependencies on libraries that are not used
  at all by dependencies of zxcrypt

Tests: added some assert_no_deps in the build, and checked the
dependencies of the new artifact:

(15:24:39) zarvox@zarvox-linux ~/fuchsia/out $ readelf -d ./default.zircon/user-x64-gcc.shlib/obj/system/dev/block/zxcrypt/zxcrypt.so | grep Shared
 0x0000000000000001 (NEEDED)             Shared library: [libzircon.so]
 0x0000000000000001 (NEEDED)             Shared library: [libdriver.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so]

DNO-492 #comment zxcrypt.so links most things statically

Change-Id: Ic7d179ae77af6e1efbaa10624ecadcd8b27134f9