GN(android): Default libs logic from runtime_library -> default_libs

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 18bd77509a5c045f587cbf0c1235fd61174b450c..694f98bc39febea784c3cb759badd2c2783ee695 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -11,6 +11,10 @@ import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/toolchain/goma.gni")
 
+if (is_android) {
+  import("//build/config/android/config.gni")
+}
+
 declare_args() {
   # When set (the default) enables C++ iterator debugging in debug builds.
   # Iterator debugging is always off in release builds (technically, this flag
@@ -363,18 +367,32 @@ config("default_libs") {
       # target that needs it.
     ]
   } else if (is_android) {
-    # Android uses -nostdlib so we need to add even libc here.
-    libs = [
-      # TODO(brettw) write a version of this, hopefully we can express this
-      # without forking out to GCC just to get the library name. The android
-      # toolchain directory should probably be extracted into a .gni file that
-      # this file and the android toolchain .gn file can share.
-      #   # Manually link the libgcc.a that the cross compiler uses.
-      #   '<!(<(android_toolchain)/*-gcc -print-libgcc-file-name)',
-      "c",
+    ldflags = [ "-nostdlib" ]
+    lib_dirs = [ "$android_libcpp_root/libs/$android_app_abi" ]
+
+    # The libc++ runtime library (must come first).
+    # ASan needs to dynamically link to libc++ even in static builds so
+    # that it can interpose operator new.
+    if (is_component_build || is_asan) {
+      libs = [ "c++_shared" ]

[brettw, 2016/01/05 18:36:44]

On Linux the difference between SDK and runtime library is a bit less clear than
on Windows. But this stuff all seems more like a runtime library than an SDK
(thinking of standard library language features vs platform features as the
distinction). I also think it's nicer to have more stuff in the android-specific
file so there's less clutter in this shared file.

"dl" should probably stay here since it's an OS feature (and the Linux one is
here below). I don't actually know what's in libatomic.

[agrieve, 2016/01/06 02:09:54]

Looks like libatomic is along the same lines as libgcc:
https://gcc.gnu.org/wiki/Atomic/GCCMM

Moved things around based on your comments.

+    } else {
+      libs = [ "c++_static" ]
+    }
+
+    # libgcc must come before libdl for ld.bfd (MIPS)
+    libs += [
+      # Manually link the libgcc.a that the cross compiler uses. This is
+      # absolute because the linker will look inside the sysroot if it's not.
+      rebase_path(android_libgcc_file),
+      "c",  # Required because of -nostdlib
       "dl",
       "m",
     ]
+
+    # Clang with libc++ does not require an explicit atomic library reference.
+    if (!is_clang) {
+      libs += [ "atomic" ]
+    }
   } else if (is_mac) {
     libs = [
       "AppKit.framework",