Move c++ configs from buildtools to build/config/c++ [src-side change]

This CL moves //buildtools/third_party/libc++:no_libstdcpp and part of
/buildtools/third_party/libc++:config into //build/config/c++/c++.
This has several advantages:
* Users can remove libc++ config if they want since it's added in
  BUILDCONFIG.gn
* Allows removal of libcxx_proxy
* libc++-specific configs are in buildtools and non-libc++ configs are
  in build/config
* Allows changing c++ flags without having to roll buildtools
* Removes a bit of duplicated build configuration

buildtools change:
https://codereview.chromium.org/2962423002

BUG=593874
R=thakis@chromium.org
TBR=dpranke@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_msan_rel_ng;master.tryserver.chromium.linux:linux_chromium_ubsan_rel_ng;master.tryserver.chromium.linux:linux_chromium_dbg_ng

Review-Url: https://codereview.chromium.org/2964243002
Cr-Commit-Position: refs/heads/master@{#486229}

DEPS

@@ -56,7 +56,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.
-  'buildtools_revision': '1dcd1bdbe93467531a50b60dbd18860803ca7be1',
+  'buildtools_revision': 'c2827ae578be603fb41a141631c27c3c09191dca',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.

build/config/BUILD.gn

@@ -272,7 +272,7 @@ group("exe_and_shlib_deps") {
     public_deps += [ "//build/config/sanitizers:deps" ]
   }
   if (use_custom_libcxx) {
-    public_deps += [ "//buildtools/third_party/libc++:libcxx_proxy" ]
+    public_deps += [ "//buildtools/third_party/libc++" ]
   }
   if (use_afl) {
     public_deps += [ "//third_party/afl" ]

build/config/c++/BUILD.gn

@@ -19,5 +19,38 @@ config("c++flags") {
       "-isystem" + rebase_path("$prefix/libc++/$include", root_build_dir),
       "-isystem" + rebase_path("$prefix/libc++abi/$include", root_build_dir),
     ]
+    if (is_linux && current_cpu == "arm") {
+      cflags_c = [ "-isystem" +
+                   rebase_path("$prefix/libunwind/$include", root_build_dir) ]
+      cflags_cc += [ "-isystem" +
+                     rebase_path("$prefix/libunwind/$include", root_build_dir) ]
+    }
+
+    # Make sure we don't link against libc++ or libstdc++.
+    ldflags = [ "-nodefaultlibs" ]
+
+    # Unfortunately, there's no way to disable linking against just
+    # libc++ (besides using clang instead of clang++); -nodefaultlibs
+    # removes all of the default libraries, so add back the ones that we
+    # need.
+    libs = [
+      "c",
+      "m",
+    ]
+
+    if (!is_mac) {
+      libs += [
+        "gcc_s",
+        "rt",
+      ]
+    }
+
+    if (is_mac && using_sanitizer) {
+      lib_dirs = [ "//third_party/llvm-build/Release+Asserts/lib/clang/$clang_version/lib/darwin" ]
+
+      if (is_asan) {
+        libs += [ "clang_rt.asan_osx_dynamic" ]
+      }
+    }
   }
 }