[lld-macho] Make test produce the dead.o and live.o that are used below.

Follow up fix to breakages in D112485

Author: oontvoo

lld/test/MachO/treat-undef-sym.s

@@ -1,7 +1,9 @@
 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 
-# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %s -o %t/live.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/live.s -o %t/live.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/dead.s -o %t/dead.o
+        
 # RUN: not %lld -undefined bogus -o /dev/null %t/live.o 2>&1 | \
 # RUN:     FileCheck %s -check-prefix=UNKNOWN
 # RUN: not %lld -undefined error -o /dev/null %t/live.o 2>&1 | \