[Flang] flang-new doesn't take --gcc-install-dir nor --gcc-toolchain

Spotted the issue when deploying `x86_64-unknown-linux-gnu.cfg` for C/C++/Fortran compilers.
```
flang-new: error: unknown argument: '--gcc-install-dir=/soft/compilers/gcc/12.2.0/x86_64-suse-linux/lib/gcc/x86_64-pc-linux-gnu/12.2.0'
```
Need at least `--gcc-install-dir` option to work.

Only the deprecated way via CMake `-DGCC_INSTALL_PREFIX` worked