Setting 'LLVM_INSTALL_DIR=/path/to/clang-_ARCH_' now searches for LLVM
in /path/to/clang-64 or /path/to/clang-32, depending on the target
architecture.

Change-Id: I555b6f345731448f66a30947d1e7d4a965caae1c
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>