Fix test/Driver/riscv32-toolchain.c for builds setting CLANG_DEFAULT_LINKER

Petr Hosek reported an external buildbot was failing on riscv32-toolchain.c, 
seemingly as it set CLANG_DEFAULT_LINKER to lld. Address this by explicitly 
setting -fuse-ld=ld in the tests.

llvm-svn: 322435