Hexagon relies on hexagon-link so invoke Clang to -fuse-ld=ld to
always use the default linker, otherwise tests break when the Clang
default linker is set to anything other than default.
Passing -fuse-ld=ld is less error prone than checking for hexagon-link or *ld* in case someone sets the default linker to something else, e.g. link.exe on Windows or something like foo-link on some new platform.