[llvm-objdump] Document --mattr=help in --help output

This does the same as `--mcpu=help` but was only
documented in the user guide.

* Added a test for both options.
* Corrected the single dash in `-mcpu=help` text.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D92305

Author: DavidSpickett

llvm/docs/CommandGuide/llvm-objdump.rst

@@ -156,7 +156,7 @@ OPTIONS
 
 .. option:: --mattr=<a1,+a2,-a3,...>
 
-  Enable/disable target-specific attributes. Specify ``--mcpu=help`` to display
+  Enable/disable target-specific attributes. Specify ``--mattr=help`` to display
   the available attributes.
 
 .. option:: --no-leading-addr

llvm/test/tools/llvm-objdump/mattr-mcpu-help.test

@@ -0,0 +1,15 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-objdump -d %t --mattr=help 2>&1 | FileCheck %s
+# RUN: llvm-objdump -d %t --mcpu=help 2>&1 | FileCheck %s
+
+# CHECK: Available CPUs for this target:
+# CHECK: Available features for this target:
+## To check we still disassemble the file:
+# CHECK: file format elf64-x86-64
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_X86_64

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -226,13 +226,13 @@ static cl::alias MachOm("m", cl::desc("Alias for --macho"), cl::NotHidden,
                         cl::Grouping, cl::aliasopt(MachOOpt));
 
 cl::opt<std::string> objdump::MCPU(
-    "mcpu", cl::desc("Target a specific cpu type (-mcpu=help for details)"),
+    "mcpu", cl::desc("Target a specific cpu type (--mcpu=help for details)"),
     cl::value_desc("cpu-name"), cl::init(""), cl::cat(ObjdumpCat));
 
-cl::list<std::string> objdump::MAttrs("mattr", cl::CommaSeparated,
-                                      cl::desc("Target specific attributes"),
-                                      cl::value_desc("a1,+a2,-a3,..."),
-                                      cl::cat(ObjdumpCat));
+cl::list<std::string> objdump::MAttrs(
+    "mattr", cl::CommaSeparated,
+    cl::desc("Target specific attributes (--mattr=help for details)"),
+    cl::value_desc("a1,+a2,-a3,..."), cl::cat(ObjdumpCat));
 
 cl::opt<bool> objdump::NoShowRawInsn(
     "no-show-raw-insn",