[Clang][VE] Fix toolchain test when -DCLANG_DEFAULT_LINKER=lld in use

The CLANG_DEFAULT_LINKER flag overrides the default toolchain linker.
VE strictly requires 'nld' to be the default linker.  This causes a test
failure in test/Driver/ve-toolchain.cpp when configured with
CLANG_DEFAULT_LINKER!=ld

  Failure in clang-ppc64le-rhel
  (https://lab.llvm.org/buildbot/#/builders/57/builds/12628)

Until default linker selection with CLANG_DEFAULT_LINKER!=ld is fixed
proper, we manually specify '-fuse-ld=ld' (ie the toolchain default
linker) in the ve-toolchain tests.