[ELF][test] Fix build break after 20bdd3e

The added run lines build a bitcode file for x86 and an object file
for whatever the default target is that is running the test. This
causes an incompatibility between the files.
Add the triple to the llvm-mc invocation.

lld/test/ELF/lto/duplicated.ll

@@ -1,7 +1,7 @@
 ; REQUIRES: x86
 ; RUN: rm -rf %t && split-file %s %t
 ; RUN: llvm-as %t/a.ll -o %t/a.bc
-; RUN: llvm-mc -filetype=obj %t/b.s -o %t/b.o
+; RUN: llvm-mc --triple=x86_64-unknown-linux-gnu -filetype=obj %t/b.s -o %t/b.o
 ; RUN: not ld.lld %t/a.bc %t/a.bc -o /dev/null -shared 2>&1 | FileCheck %s
 
 ;; --thinlto-index-only skips some passes. Test the error is present.