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>

cmake/compiler/clang/target.cmake

@@ -32,6 +32,10 @@ if(NOT "${ARCH}" STREQUAL "posix")
     include(${ZEPHYR_BASE}/cmake/compiler/clang/target_arm.cmake)
   endif()
 
+  if(DEFINED CMAKE_C_COMPILER_TARGET)
+    set(clang_target_flag "--target=${CMAKE_C_COMPILER_TARGET}")
+  endif()
+
   foreach(file_name include/stddef.h)
     execute_process(
       COMMAND ${CMAKE_C_COMPILER} --print-file-name=${file_name}
@@ -57,7 +61,8 @@ if(NOT "${ARCH}" STREQUAL "posix")
 
   # This libgcc code is partially duplicated in compiler/*/target.cmake
   execute_process(
-    COMMAND ${CMAKE_C_COMPILER} ${TOOLCHAIN_C_FLAGS} --print-libgcc-file-name
+    COMMAND ${CMAKE_C_COMPILER} ${clang_target_flag} ${TOOLCHAIN_C_FLAGS}
+            --print-libgcc-file-name
     OUTPUT_VARIABLE RTLIB_FILE_NAME
     OUTPUT_STRIP_TRAILING_WHITESPACE
     )