[llvm-mc-assemble-fuzzer] Initialize MCTargetOptions.

[HsiangKai]

Summary

When run the command in the llvm-mc-assemble-fuzzer document,

llvm-mc-fuzzer --triple=aarch64-linux-gnu --fuzzer-args -max_len=4

it triggers the following assertion:

llvm-mc-assemble-fuzzer:
llvm-project/llvm/lib/MC/MCTargetOptionsCommandFlags.cpp:38:
bool llvm::mc::getRelaxAll(): Assertion `RelaxAllView &&
"RegisterMCTargetOptionsFlags not created."' failed.

It is caused by no global RegisterMCTargetOptionsFlags object to initialize
the MC target options.

[xgupta]

I can confirm the issue and the patch is also fixed the assertion. So LGTM.

llvm/tools/llvm-mc-assemble-fuzzer/llvm-mc-assemble-fuzzer.cpp

40

MCTargetOptionsFlags -> MOF
(shorter name might be better? as used in ./llvm/tools/llvm-mca/llvm-mca.cpp:69 or ./llvm/tools/llvm-mc/llvm-mc.cpp:44)

llvm/tools/llvm-mc-assemble-fuzzer/llvm-mc-assemble-fuzzer.cpp

 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/ToolOutputFile.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
+static mc::RegisterMCTargetOptionsFlags MOF;
+
 static cl::opt<std::string>
     TripleName("triple", cl::desc("Target triple to assemble for, "
                                   "see -version for available targets"));
 
 static cl::opt<std::string>
     MCPU("mcpu",
          cl::desc("Target a specific cpu type (-mcpu=help for details)"),
          cl::value_desc("cpu-name"), cl::init(""));