[flatbuffers] Add cross-compilation support for flatc

[ras0219-msft]

flatbuffers produces both a runtime library and a code generator flatc. While it does not run flatc during its own build, consumers expect to find the tool via the import Flatbuffers:flatc.

In this PR, I have made the cross build of flatbuffers import the targets from the host build, enabling seamless access for consumers. I tested this with apsi, however it fails during compilation when targeting arm64-windows for other reasons.

[github-actions]

You have modified or added at least one portfile where deprecated functions are used.

If you feel able to do so, please consider migrating them to the new functions:
vcpkg_install_cmake -> vcpkg_cmake_install (from port vcpkg-cmake)
vcpkg_build_cmake -> vcpkg_cmake_build (from port vcpkg-cmake)
vcpkg_configure_cmake -> vcpkg_cmake_configure (Please remove the option PREFER_NINJA) (from port vcpkg-cmake)
vcpkg_fixup_cmake_targets -> vcpkg_cmake_config_fixup (from port vcpkg-cmake-config)

In the ports that use the new function, you have to add the corresponding dependencies:

{
  "name": "vcpkg-cmake",
  "host": true
},
{
  "name": "vcpkg-cmake-config",
  "host": true
}

The following files are affected:

• ports/flatbuffers/portfile.cmake

You have modified or added at least one vcpkg.json where a "license" field is missing.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/flatbuffers/vcpkg.json

Valid values for the license field can be found in the documentation

[github-actions]

You have modified or added at least one portfile where deprecated functions are used.

If you feel able to do so, please consider migrating them to the new functions:
vcpkg_install_cmake -> vcpkg_cmake_install (from port vcpkg-cmake)
vcpkg_build_cmake -> vcpkg_cmake_build (from port vcpkg-cmake)
vcpkg_configure_cmake -> vcpkg_cmake_configure (Please remove the option PREFER_NINJA) (from port vcpkg-cmake)
vcpkg_fixup_cmake_targets -> vcpkg_cmake_config_fixup (from port vcpkg-cmake-config)

In the ports that use the new function, you have to add the corresponding dependencies:

{
  "name": "vcpkg-cmake",
  "host": true
},
{
  "name": "vcpkg-cmake-config",
  "host": true
}

The following files are affected:

• ports/flatbuffers/portfile.cmake

You have modified or added at least one vcpkg.json where a "license" field is missing.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/flatbuffers/vcpkg.json

Valid values for the license field can be found in the documentation

[github-actions]

You have modified or added at least one vcpkg.json where a "license" field is missing.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/flatbuffers/vcpkg.json

Valid values for the license field can be found in the documentation

[BillyONeal]

Thanks for the fix!