gn build: Add support for building lldb-server on Android.

[pcc]

Summary

The cross-compiled lldb-server targets are added to the top-level
lldb target if Android cross compilation is enabled.

[thakis]

LGTM

[pcc]

Move to data_deps

llvm/utils/gn/build/libs/xml/enable.gni

 declare_args() {
   # Whether to include code that links against libxml2.
-  llvm_enable_libxml2 = host_os != "win"
+  llvm_enable_libxml2 = current_os != "win" && current_os != "android"
 }

llvm/utils/gn/secondary/lldb/tools/driver/BUILD.gn

 import("//llvm/utils/TableGen/tablegen.gni")
+import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 import("//llvm/version.gni")
 
 tablegen("Options") {
   visibility = [ ":lldb" ]
   args = [ "-gen-opt-parser-defs" ]
 }
 
 if (current_os == "mac") {
   write_cmake_config("write_info_plist") {
     input = "lldb-Info.plist.in"
     output = "$target_gen_dir/lldb-Info.plist"
     values = [ "LLDB_VERSION=$llvm_version" ]
   }
 }
 
+supported_toolchains = []
+if (android_ndk_path != "") {
+  supported_toolchains += [
+    "//llvm/utils/gn/build/toolchain:stage2_android_aarch64",
+    "//llvm/utils/gn/build/toolchain:stage2_android_arm",
+  ]
+}
+
 executable("lldb") {
   configs += [ "//llvm/utils/gn/build:lldb_code" ]
   deps = [
     ":Options",
     "//lldb/include/lldb/Host:Config",
     "//lldb/source/API:liblldb",
     "//llvm/lib/Option",
     "//llvm/lib/Support",
     "//llvm/utils/gn/build/libs/xar",
   ]
   if (current_os == "mac") {
     deps += [ ":write_info_plist" ]
     plist = get_target_outputs(":write_info_plist")
     ldflags = [ "-Wl,-sectcreate,__TEXT,__info_plist," +
                 rebase_path(plist[0], root_out_dir) ]
   }
   data_deps = []
   if (current_os == "linux") {
     data_deps += [ "//lldb/tools/lldb-server" ]
   }
+  foreach (toolchain, supported_toolchains) {
+    data_deps += [ "//lldb/tools/lldb-server($toolchain)" ]
+  }
 
   sources = [
     "Driver.cpp",
     "Platform.cpp",
   ]
 
   # XXX if win -DIMPORT_LIBLLDB
   # XXX LLDB_BUILD_FRAMEWORK rpath stuff
 }

llvm/utils/gn/secondary/lldb/tools/lldb-server/BUILD.gn

+import("//compiler-rt/target.gni")
 import("//llvm/utils/TableGen/tablegen.gni")
 
 tablegen("LLGSOptions") {
   visibility = [ ":lldb-server" ]
   args = [ "-gen-opt-parser-defs" ]
 }
 
 executable("lldb-server") {
executable("lldb-server") {
   sources = [
     "Acceptor.cpp",
     "LLDBServerUtilities.cpp",
     "SystemInitializerLLGS.cpp",
     "lldb-gdbserver.cpp",
     "lldb-platform.cpp",
     "lldb-server.cpp",
   ]
+
+  if (current_os == "android") {
+    output_dir = "$root_build_dir/runtimes_ndk_cxx/$crt_current_target_arch"
+  }
 }