[gn build] Add rules for crtbegin/end (#66012)

llvm/utils/gn/secondary/compiler-rt/lib/BUILD.gn

@@ -5,7 +5,10 @@ group("lib") {
     "//compiler-rt/lib/cfi:ignorelist($host_toolchain)",
   ]
   if (current_os == "linux") {
-    deps += [ "//compiler-rt/lib/msan" ]
+    deps += [
+      "//compiler-rt/lib/builtins:crt",
+      "//compiler-rt/lib/msan",
+    ]
   }
   if (current_os == "linux" || current_os == "android") {
     deps += [ "//compiler-rt/lib/ubsan_minimal" ]

llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn

@@ -574,6 +574,38 @@ static_library("builtins") {
   deps = lse_targets
 }
 
+if (current_os == "linux") {
+  source_set("crt_src") {
+    sources = [
+      "crtbegin.c",
+      "crtend.c",
+    ]
+    cflags = [
+      "-std=c11",
+      "-fPIC",
+    ]
+  }
+  copy("crtbegin") {
+    # TODO: use get_target_outputs if it ever works with source_set to avoid hardcoding crt_src.crtbegin.o
+    input_dir = get_label_info(":crt_src", "target_out_dir")
+    sources = [ "$input_dir/crt_src.crtbegin.o" ]
+    outputs = [ "$crt_current_out_dir/clang_rt.crtbegin.o" ]
+    deps = [ ":crt_src" ]
+  }
+  copy("crtend") {
+    input_dir = get_label_info(":crt_src", "target_out_dir")
+    sources = [ "$input_dir/crt_src.crtend.o" ]
+    outputs = [ "$crt_current_out_dir/clang_rt.crtend.o" ]
+    deps = [ ":crt_src" ]
+  }
+  group("crt") {
+    deps = [
+      ":crtbegin",
+      ":crtend",
+    ]
+  }
+}
+
 # Currently unused but necessary to make sync_source_lists_from_cmake.py happy.
 source_set("_unused") {
   sources = [