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/BUILD.gn

+import("//llvm/utils/gn/build/toolchain/compiler.gni")
+
+supported_toolchains = []
+if (android_ndk_path != "") {
+  supported_toolchains += [
+    "//llvm/utils/gn/build/toolchain:stage2_android_aarch64",
+    "//llvm/utils/gn/build/toolchain:stage2_android_arm",
+  ]
+}
+
 group("lldb") {
   deps = [ "tools/driver:lldb" ]
   if (current_os == "linux") {
     deps += [ "tools/lldb-server" ]
   }
+  foreach (toolchain, supported_toolchains) {
+    deps += [ "tools/lldb-server($toolchain)" ]
+  }
 }

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"
+  }
 }