clang: Provide --target option when determining path to runtime library

Clang/LLVM is natively a cross-compiler, so one set of applications can
compile code to all supported targets. The default target can be changed
using '--target' option.

CMake supports this type of compilers. To change compiling target, one
should set CMAKE_C_COMPILER_TARGET accorgindly.

The '--target' option has impact on the path to clang-rt library
returned by compiler when run with '--print-libgcc-file-name' option.
Without specifying target, Clang will return path to runtime library of
the host target (e.g. x86_64-pc-linux-gnu).

Signed-off-by: Patryk Duda <pdk@semihalf.com>