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.

llvm-svn: 225182