[RISCV] Explicitly set an empty --sysroot in the test

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).

Differential Revision: https://reviews.llvm.org/D51972

llvm-svn: 342060