Build: Offsets lib as OBJECT not STATIC

With the new cmake version, we are able to simplify the generation of
the offset object library without using absolute hardcoded paths.

Instead of the hardcoded paths, we now use the generator:
$<TARGET_OBJECTS:${OFFSETS_LIB}>

This is needed to eventually be able to have an arch-out-of-tree build

Signed-off-by: Klaus Petersen <kape@oticon.com>

CMakeLists.txt

@@ -691,10 +691,9 @@ gen_kobj(KOBJ_INCLUDE_PATH)
 set(OFFSETS_LIB offsets)
 
 set(OFFSETS_C_PATH ${ZEPHYR_BASE}/arch/${ARCH}/core/offsets/offsets.c)
-set(OFFSETS_O_PATH ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/${OFFSETS_LIB}.dir/arch/${ARCH}/core/offsets/offsets.c.obj)
 set(OFFSETS_H_PATH ${PROJECT_BINARY_DIR}/include/generated/offsets.h)
 
-add_library(          ${OFFSETS_LIB} STATIC ${OFFSETS_C_PATH})
+add_library(          ${OFFSETS_LIB} OBJECT ${OFFSETS_C_PATH})
 target_link_libraries(${OFFSETS_LIB} zephyr_interface)
 add_dependencies(     ${OFFSETS_LIB}
   ${SYSCALL_LIST_H_TARGET}
@@ -706,7 +705,7 @@ add_dependencies(     ${OFFSETS_LIB}
 add_custom_command(
   OUTPUT ${OFFSETS_H_PATH}
   COMMAND ${PYTHON_EXECUTABLE} ${ZEPHYR_BASE}/scripts/gen_offset_header.py
-  -i ${OFFSETS_O_PATH}
+  -i $<TARGET_OBJECTS:${OFFSETS_LIB}>
   -o ${OFFSETS_H_PATH}
   DEPENDS ${OFFSETS_LIB}
 )
@@ -881,7 +880,7 @@ set(zephyr_lnk
   ${ZEPHYR_LIBS_PROPERTY}
   ${LINKERFLAGPREFIX},--no-whole-archive
   kernel
-  ${OFFSETS_O_PATH}
+  $<TARGET_OBJECTS:${OFFSETS_LIB}>
   ${LIB_INCLUDE_DIR}
   -L${PROJECT_BINARY_DIR}
   ${TOOLCHAIN_LIBS}