Fix clash of gcc toolchains in driver regression tests

For some regression tests the path to the right toolchain is specified
using the -sysroot switch. However, if clang was configured with a
custom gcc toolchain (either by using GCC_INSTALL_PREFIX in cmake or the
equivalent configure command), the path to the custom gcc toolchain path
takes precedence to the one specified by sysroot. This causes several
regression tests to fail as they will be using an unexpected path. This
patch fixes this issue by adding --gcc-toolchain='' to all tests that
rely on that. The empty string causes the driver to pick the path from
sysroot instead.

This patch contain the same kind of fixes as done in rC225182

llvm-svn: 339112