[llvm] fix bitcode-strip.test on windows

Remove the executable name from the test match as this will have
a `.exe` suffix on windows.

Reviewed By: drodriguez

Differential Revision: https://reviews.llvm.org/D121000

llvm/test/tools/llvm-objcopy/MachO/bitcode-strip.test

@@ -4,7 +4,7 @@
 # RUN: llvm-bitcode-strip %t -o %t2
 # RUN: cmp %t %t2
 
-# MISSING-ARG: llvm-bitcode-strip: error: -o is a required argument
+# MISSING-ARG: error: -o is a required argument
 
 --- !mach-o
 FileHeader: