devel/llvm13: fix BE_NATIVE build

Handle new per-backend Exegesis libraries. Awkwardly, they use the
opposite convention of all other per-arch binaries so add (hopefuly
temporary) machinery to support libLLVM<Foo><Arch>.a libraries.

PR: 258897
Sponsored by: DARPA

(cherry picked from commit 1fca16446ff2ff675ee3cfd6284fa46016a7d498)