[BOLT] Added fuzzer target (llvm-bolt-fuzzer)

This adds a target that would consume random binary as an
input ELF file.
TBD: add structured input support (ELF).

Build:
```
cmake /path/to/llvm-project/llvm -GNinja \
-DLLVM_TARGETS_TO_BUILD="X86;AArch64" \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_ASSERTIONS=1 \
-DCMAKE_C_COMPILER=<sanitizer-capable clang> \
-DCMAKE_CXX_COMPILER=<sanitizer-capable clang++> \
-DLLVM_ENABLE_PROJECTS="bolt"  \
-DLLVM_USE_SANITIZER=Address \
-DLLVM_USE_SANITIZE_COVERAGE=On
ninja llvm-bolt-fuzzer
```

Test Plan: ninja llvm-bolt-fuzzer

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D120016