Add -Xcc and -gcc options to dmd-script.

[dbankov-broadcom]

Add -Xcc and -gcc options to better support bootstrapping of LDC.
Having -Xcc and -gcc options is also useful in general when having to
link D and C/C++ object files.

[dbankov-broadcom]

BTW this PR is the result of the discussion in ldc-developers/ldc#3810 PR. This change will allow bootstrapping LDC with statically linked GDRuntime and GPhobos by using:

cmake -G Ninja <ldc-src-dir> -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=$PWD/../install -DD_COMPILER=gdmd-11 -DD_COMPILER_ID=GDMD -DD_COMPILER_FE_VERSION=2076 -DD_COMPILER_FLAGS=-gcc=g++-11 -DCMAKE_C_COMPILER=gcc-11 -DCMAKE_CXX_COMPILER=g++-11 -DLDC_LINK_MANUALLY=OFF

The above will unfortunately result in the symbol conflicts mentioned in ldc-developers/ldc#3810 unless a version of GCC which has the corresponding symbols marked as weak is used (I'll post a PR with this change shortly). Passing -q,-shared-libphobos (see below) could be used as a workaround until GDRuntime and GPhobos are updated to have the corresponding symbols marked as weak in the upstream GCC repository.

cmake -G Ninja <ldc-src-dir> -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=$PWD/../install -DD_COMPILER=gdmd-11 -DD_COMPILER_ID=GDMD -DD_COMPILER_FE_VERSION=2076 -DD_COMPILER_FLAGS="-gcc=g++-11 -q,-shared-libphobos" -DCMAKE_C_COMPILER=gcc-11 -DCMAKE_CXX_COMPILER=g++-11 -DLDC_LINK_MANUALLY=OFF