In rL341655 we added additional behaviour to the Driver for riscv32-unknown-elf when the sysroot is empty.
The new tests that check the new behaviour expect that the absence of --sysroot in the command-line implies that the sysroot empty. This doesn't hold if clang is built with a non-empty DEFAULT_SYSROOT in cmake. When this is the case, this test fails.
Since the new behaviour is triggered when the sysroot is empty, pass an empty --sysroot to avoid using the default (if any).