[RISCV] Improve sysroot computation if no GCC install detected

If a GCC installed is not detected, the driver would default to
the root of the filesystem. This is not ideal when this doesn't
match the install directory of the toolchain and can cause
undesireable behavior such as picking up system libraries or
the system linker when cross-compiling.

Differential Revision: https://reviews.llvm.org/D68391