Clang choses wrong sysroot

I have built clang-13 with target RISCV and X86…
My system native architecture is X86 so when I compile with --target=riscv64-unknown-linux-gnu I also specify -isysroot=/projects/riscv64-gnu-linux/cross/riscv64-linux/ where I installed linux headers for riscv.
However when I invoke clang I see the following:
"/projects/riscv64-gnu-linux/cross/clang-13.0.0/bin/clang-13" -cc1 -triple riscv64-unknown-linux-gnu -emit-obj -mrelax-all --mrelax-relocations -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.cpp -mrelocation-model static -mframe-pointer=all -fmath-errno -fno-rounding-math -mconstructor-aliases -target-cpu sifive-u74 -target-feature +m -target-feature +a -target-feature +f -target-feature +d -target-feature +c -target-feature +64bit -target-feature +relax -target-feature -save-restore -target-abi lp64d -msmall-data-limit 8 -debugger-tuning=gdb -v -fcoverage-compilation-dir=/projects/riscv64-gnu-linux/tests/hello_world -resource-dir /projects/riscv64-gnu-linux/cross/clang-13.0.0/lib/clang/13.0.0 -isysroot =/projects/riscv64-gnu-linux/cross/riscv64-linux/ -I /projects/riscv64-gnu-linux/cross/clang-13.0.0/include/c++/v1 -internal-isystem /projects/riscv64-gnu-linux/cross/clang-13.0.0/lib/clang/13.0.0/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir=/projects/riscv64-gnu-linux/tests/hello_world -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/main-97d97e.o -x c++ main.cpp

Note the -internal that clang adds and that point to the x86 linux headers instead of just using the sysroot I specified.
I then run into issues with missing headers due to the mismatch of architecture/headers.
How can I convince clang that it should really not rely on default system headers and believe me when I specify sysroot?

It would seem that -isysroot was not the right approach, using --sysroot instead fixed that problem. Now I just need to understand why clang does not recognize my gcc installation and I should be able to simplify my compile line significantly.