[binutils] Add response file option to help and docs

Many LLVM-based tools already support response files (i.e. files
containing a list of options, specified with '@'). This change simply
updates the documentation and help text for some of these tools to
include it. I haven't attempted to fix all tools, just a selection that
I am interested in.

I've taken the opportunity to add some tests for --help behaviour, where
they were missing. We could expand these tests, but I don't think that's
within scope of this patch.

This fixes https://bugs.llvm.org/show_bug.cgi?id=42233 and
https://bugs.llvm.org/show_bug.cgi?id=42236.

Reviewed by: grimar, MaskRay, jkorous

Differential Revision: https://reviews.llvm.org/D63597

llvm-svn: 364036
diff --git a/llvm/tools/llvm-size/llvm-size.cpp b/llvm/tools/llvm-size/llvm-size.cpp
index e422e69..89b89f0 100644
--- a/llvm/tools/llvm-size/llvm-size.cpp
+++ b/llvm/tools/llvm-size/llvm-size.cpp
@@ -99,6 +99,9 @@
 static cl::list<std::string>
     InputFilenames(cl::Positional, cl::desc("<input files>"), cl::ZeroOrMore);
 
+static cl::extrahelp
+    HelpResponse("\nPass @FILE as argument to read options from FILE.\n");
+
 static bool HadError = false;
 
 static std::string ToolName;