[RISCV] Fix sysroot tests without GCC on RISC-V hosts with GCC

[RISCV] Fix sysroot tests without GCC on RISC-V hosts with GCC

D68391 added tests that check scenarios where no RISC-V GCC toolchain is
supposed to be detected. When running the tests on RISC-V hosts the system's
GCC toolchain will be detected, and the tests will fail. This patch adds a
`--gcc-toolchain` option pointing to a path where no GCC toolchain is
present, ensuring that the tests are run under the expected conditions, and
therefore are able to pass in all test environments.

Differential Revision: https://reviews.llvm.org/D75061