[libc][cmake] append per obj compile options instead of prepending (llvm#77126)

This allows individual object files to override the common compile
commands in
their local CMakeLists' add_object_library call.

For example, the common compile commands contain -Wall and -Wextra.
Before
this patch, the per object COMPILE_OPTIONS were prepended to these, so
that
builds of individual object files could not individually disable
specific
diagnostics from those groups explicitly.

After this patch, the per-object file compile objects are appended to
the list
of compiler flags, enabling this use case.

ARGN is a bit of cmake magic; let's be explicit in the APPEND that we're
appending the compile options.

Link: llvm#77007

Author: nickdesaulniers

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -26,7 +26,7 @@ function(_get_common_compile_options output_var flags)
     set(ADD_PREFER_GENERIC_FLAG TRUE)
   endif()
 
-  set(compile_options ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ARGN})
+  set(compile_options ${LIBC_COMPILE_OPTIONS_DEFAULT})
   if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
     list(APPEND compile_options "-fpie")
 
@@ -357,11 +357,8 @@ function(create_object_library fq_target_name)
     set(internal_target_name ${fq_target_name})
   endif()
 
-  _get_common_compile_options(
-    compile_options
-    "${ADD_OBJECT_FLAGS}"
-    ${ADD_OBJECT_COMPILE_OPTIONS}
-  )
+  _get_common_compile_options(compile_options "${ADD_OBJECT_FLAGS}")
+  list(APPEND compile_options ${ADD_OBJECT_COMPILE_OPTIONS})
 
   # GPU builds require special handling for the objects because we want to
   # export several different targets at once, e.g. for both Nvidia and AMD.
@@ -640,11 +637,8 @@ function(create_entrypoint_object fq_target_name)
     set(ADD_ENTRYPOINT_OBJ_CXX_STANDARD ${CMAKE_CXX_STANDARD})
   endif()
 
-  _get_common_compile_options(
-    common_compile_options
-    "${ADD_ENTRYPOINT_OBJ_FLAGS}"
-    ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS}
-  )
+  _get_common_compile_options(common_compile_options "${ADD_ENTRYPOINT_OBJ_FLAGS}")
+  list(APPEND common_compile_options ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS})
   get_fq_deps_list(fq_deps_list ${ADD_ENTRYPOINT_OBJ_DEPENDS})
   set(full_deps_list ${fq_deps_list} libc.src.__support.common)