Fix issue with clang --target when cross compiling using LLVM

Fix issue with clang --target when cross compiling using LLVM

When using LLVM and cross compiling, the CC expands to:
clang --target=<target_triple>
Example of target triple: aarch64-linux-gnu.

However the current code assigning --target based on the
cross compiler does not work if we pass the full path to
CROSS_COMPILE. In this case the build fails.

Fix the issue so that when cross compiling using LLVM
only the basename of cross compiler path is used.

Closes #2

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>