On Fuchsia, we can now get executable VMOs from trusted backing
filesystems. This allows us to remove the use of replace_as_executable
in favor of opening files with `fdio_open_fd_at` with the
`OPEN_RIGHT_EXECUTABLE` flag and getting VMOs by calling
`fdio_get_vmo_exec`.
By moving the responsibility for executability into the filesystem, we
should be able to remove deprecated-ambient-replace-as-executable from
component manifests for non-JIT runners (the JIT runners still call
replace_as_executable in Dart's allocator).
Test: verified locally that this works on Astro on a _user build with
the runtime allowlist tightened.
ff921cd608