bootstrap.bat does not respect overridden gcc toolset

Hi,

Due to msys2/MINGW-packages#10800 and https://github.com/bfgroup/b2/issues/138, I need to add a `user-config.jam` to bootstrap b2 correctly:

```
using gcc : : D:/msys64/clang64/bin/clang++.exe ;
```

However, the bootstrap batch file for boost completely ignores the overridden `gcc` toolset:

https://github.com/boostorg/boost/blob/2b06d9f20560a4f7292833fcb71d8c6a3652ca76/bootstrap.bat#L49-L59

whereas the Bash version does not:

https://github.com/boostorg/boost/blob/2b06d9f20560a4f7292833fcb71d8c6a3652ca76/bootstrap.sh#L337-L354

so I need to yank project-config.jam prior to launching b2.