cmake: toolchain: Change the names of 'clang' and 'llvm'

The 'llvm' and 'clang' build scripts have been named strangely. It is
more natural for 'clang' to be the compiler, and 'llvm' to be the
toolchain.

This commit rectifies this by renaming the files.

This also fixes #11187

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
2 files changed