[libc] [startup] add cmake function to merge separated crt1 objects (llvm#75413)

As part of startup refactoring, this patch adds a function to merge
multiple objects into a single relocatable object:
                     cc -r obj1.o obj2.o -o obj.o

A relocatable object is an object file that is not fully linked into an
executable or a shared library. It is an intermediate file format that
can be passed into the linker.

A crt object can have arch-specific code and arch-agnostic code. To
reduce code cohesion, the implementation is splitted into multiple
units. As a result, we need to merge them into a single relocatable
object.

Author: SchrodingerZhu

libc/startup/linux/CMakeLists.txt

@@ -1,19 +1,62 @@
+# This function merges multiple objects into a single relocatable object
+#                     cc -r obj1.o obj2.o -o obj.o
+# A relocatable object is an object file that is not fully linked into an
+# executable or a shared library. It is an intermediate file format that can
+# be passed into the linker.
+# A crt object has arch-specific code and arch-agnostic code. To reduce code
+# duplication, the implementation is split into multiple units. As a result,
+# we need to merge them into a single relocatable object.
+# See also:  https://maskray.me/blog/2022-11-21-relocatable-linking
+function(merge_relocatable_object name)
+  set(obj_list "")
+  set(fq_link_libraries "")
+  get_fq_deps_list(fq_dep_list ${ARGN})
+  foreach(target IN LISTS fq_dep_list)
+    list(APPEND obj_list "$<TARGET_OBJECTS:${target}>")
+    get_target_property(libs ${target} DEPS)
+    list(APPEND fq_link_libraries "${libs}")
+  endforeach()
+  list(REMOVE_DUPLICATES obj_list)
+  list(REMOVE_DUPLICATES fq_link_libraries)
+  get_fq_target_name(${name} fq_name)
+  set(relocatable_target "${fq_name}.__relocatable__")
+  add_executable(
+    ${relocatable_target}
+    ${obj_list}
+  )
+  # Pass -r to the driver is much cleaner than passing -Wl,-r: the compiler knows it is
+  # a relocatable linking and will not pass other irrelevant flags to the linker.
+  target_link_options(${relocatable_target} PRIVATE -r)
+  set_target_properties(
+    ${relocatable_target}
+    PROPERTIES
+      RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+      OUTPUT_NAME ${name}.o
+  )
+  add_library(${fq_name} OBJECT IMPORTED GLOBAL)
+  add_dependencies(${fq_name} ${relocatable_target})
+  target_link_libraries(${fq_name} INTERFACE ${fq_link_libraries})
+  set_target_properties(
+    ${fq_name} 
+    PROPERTIES
+      LINKER_LANGUAGE CXX
+      IMPORTED_OBJECTS ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
+      TARGET_TYPE ${OBJECT_LIBRARY_TARGET_TYPE}
+      DEPS "${fq_link_libraries}"
+  ) 
+endfunction()
+
 function(add_startup_object name)
   cmake_parse_arguments(
     "ADD_STARTUP_OBJECT"
-    "ALIAS" # Option argument
+    "" # Option argument
     "SRC"   # Single value arguments
     "DEPENDS;COMPILE_OPTIONS" # Multi value arguments
     ${ARGN}
   )
 
   get_fq_target_name(${name} fq_target_name)
-  if(ADD_STARTUP_OBJECT_ALIAS)
-    get_fq_deps_list(fq_dep_list ${ADD_STARTUP_OBJECT_DEPENDS})
-    add_library(${fq_target_name} ALIAS ${fq_dep_list})
-    return()
-  endif()
-
+  
   add_object_library(
     ${name}
     SRCS ${ADD_STARTUP_OBJECT_SRC}
@@ -27,18 +70,24 @@ function(add_startup_object name)
   )
 endfunction()
 
+check_cxx_compiler_flag("-r" LIBC_LINKER_SUPPORTS_RELOCATABLE)
+
+if(NOT LIBC_LINKER_SUPPORTS_RELOCATABLE)
+  message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}: linker does not support -r")
+  return()
+endif()
+
 if(NOT (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE}))
   message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}")
   return()
 endif()
 
 add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
 
-add_startup_object(
+# TODO: factor out crt1 into multiple objects
+merge_relocatable_object(
   crt1
-  ALIAS
-  DEPENDS
-    .${LIBC_TARGET_ARCHITECTURE}.crt1
+  .${LIBC_TARGET_ARCHITECTURE}.crt1
 )
 
 add_startup_object(