Bug#1040750: please add loong64 arch support in lintian

[Drew Parsons <dparsons@debian.org>]

Package: lintian
Version: 2.116.3
Followup-For: Bug #1040750

Related to this bug, the invalid-arch-string-in-source-relation needs
to be updated for loong64.

If loong64 is named as an arch-specific dependency (e.g. for packages
which are only available on 64-bit architectures), then lintian emits
an error case of invalid-arch-string-in-source-relation

e.g.
E: fenics-dolfinx source: invalid-arch-string-in-source-relation loong64 [Build-Depends: libadios2-mpi-c++11-dev [amd64 arm64 mips64el ppc64el riscv64 s390x loong64 ppc64 sparc64]]