tunes: set valid clang tune for big.LITTLE Arm SoCs

tunes: set valid clang tune for big.LITTLE Arm SoCs

clang doesn't have Arm big.LITTLE specific tune options - when such an option is used,
the compilation fails with an error like this:

aarch64-poky-linux-clang: error: unsupported argument 'cortex-a72.cortex-a53+crc+crypto' to option '-mcpu='

To avoid this, in case a big.LITTLE SoC is the target and the toolchain is clang, select
the tune corresponding to the LITTLE core.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>