Currently VMOs on Fuchsia are allocated with execute rights, but soon
that will not be the case, so the calls later to re-map memory as
executable will fail. Add zx_vmo_replace_as_executable call to ensure
they don't.

Change-Id: I45c5f5527bc36ed898b29095788006239bb2b0b3
Reviewed-on: https://swiftshader-review.googlesource.com/c/24188Reviewed-by: Wez <wez@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Matthew Dempsky <mdempsky@google.com>