[fbl] Remove libzx dependency from libfbl

Code that depends on the Zircon syscalls should be in fzl instead.
See <https://fuchsia.googlesource.com/zircon/+/master/docs/cxx.md#fzl>

Test: fzl-test
Change-Id: Ic65425fc51136edf4d401097f6b483d56923104e