Fix detection of clang TOOLCHAIN_DIR
Fix detection of clang TOOLCHAIN_DIR

At the moment, the build system assumes that clang is located
at the same path as gcc, which is not the case on Travis for
instance, resulting in a failing build.

Resolves: LE-7627
Change-Id: Ia876bf82ebae02cbff035f17385622dd3b9f2c3f

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af