[llvm-objcopy][NFC] Consistenly use two dashes for flags in tests.

Summary:
As pointed out in D53667, our use of hyphens in flags can be inconsistent, mixing `-` with `--`. This change makes all long style flags use `--`.

Automatically changed via:

```
find test/tools/llvm-objcopy/ELF -type f | xargs sed -i 's/ -\([a-zA-Z]\{3\}\)/ --\1/g'
```

Two false positives were manually fixed/reverted.

Reviewers: jhenderson, espindola, alexshap

Reviewed By: jhenderson

Subscribers: emaste, javed.absar, arichardson, fedor.sergeev, jakehehrlich, llvm-commits

Differential Revision: https://reviews.llvm.org/D56513

llvm-svn: 350944
diff --git a/llvm/test/tools/llvm-objcopy/ELF/binary-input.test b/llvm/test/tools/llvm-objcopy/ELF/binary-input.test
index 3e9b1ba..d7a708d 100644
--- a/llvm/test/tools/llvm-objcopy/ELF/binary-input.test
+++ b/llvm/test/tools/llvm-objcopy/ELF/binary-input.test
@@ -2,7 +2,7 @@
 # Preserve input to verify it is not modified
 # RUN: cp %t.x-txt %t-copy.txt
 # RUN: llvm-objcopy -I binary -B i386:x86-64 %t.x-txt %t.o
-# RUN: llvm-readobj -sections -symbols %t.o | FileCheck %s
+# RUN: llvm-readobj --sections --symbols %t.o | FileCheck %s
 # RUN: cmp %t.x-txt %t-copy.txt
 
 # CHECK:      Sections [