[BOLT][CMAKE] Use BOLT_CLANG_EXE and BOLT_LLD_EXE as is

Add an ability to provide paths that don't match tool name exactly:
e.g. clang-13.
Remove use_lld call that sets up unused extra tools.

Test plan:
```
cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt" -DBOLT_CLANG_EXE=/usr/bin/clang-13 -DBOLT_LLD_EXE=/usr/bin/lld-13
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /usr/bin/clang-13
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /usr/bin/lld-13

cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt;lld" -DBOLT_CLANG_EXE=/usr/bin/clang-13
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /usr/bin/clang-13
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /data/llvm-build2/bin/lld

cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt;clang;lld"
...
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using clang: /data/llvm-build3/bin/clang
llvm-lit: /data/llvm-project/llvm/utils/lit/lit/llvm/config.py:436: note: using lld: /data/llvm-build3/bin/lld
```

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D117446