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

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.

Differential Revision: https://reviews.llvm.org/D106417