Bug#1059090: elpi: add build support for loongarch64

[zhangdandan <zhangdandan@loongson.cn>]

Source: elpi
Version: 8.5.4-1
Severity: wishlist
Tags: patch
User: debian-loongarch@lists.debian.org
Usertags: loong64

Dear maintainers,

The elpi source package lacks LoongArch architecture support.
We need to add build support for loongarch64 in d/control, for examples,
```
Package: libelpi-ocaml
-Architecture: amd64 arm64 i386 ppc64el ppc64 riscv64 sh4
+Architecture: amd64 arm64 i386 loong64 ppc64el ppc64 riscv64 sh4

Package: libelpi-ocaml-dev
-Architecture: amd64 arm64 i386 ppc64el ppc64 riscv64 sh4
+Architecture: amd64 arm64 i386 loong64 ppc64el ppc64 riscv64 sh4

Package: elpi
-Architecture: amd64 arm64 i386 ppc64el ppc64 riscv64 sh4
+Architecture: amd64 arm64 i386 loong64 ppc64el ppc64 riscv64 sh4
```
I would like to remind you that the compilation dependencies of loong64 
elpi are not yet satisfied.
If you have any questions, you can contact me at any time.

thanks,
Dandan Zhang