Use custom libcxx by default on Linux and fix sanitizers

This CL:
 - switches Linux to libcxx by default.
 - turns libcxx into a DSO for sanitizers.
 - fixes a bunch of sanitizers build files.
 - enables the sanitizers cross-checks on the CI.

Background:
There are mainly two C++ libraries around in the world: (i) GNU's
libstdc++ and LLVM's libc++ (aka libcxx). By default Linux provides libstdc++
(even building with clang on Linux uses that by default) while Mac and
Android switched to libcxx.
buildtools/libcxx(abi) contains a fixed version of the libcxx, the same one
that Chrome uses on most production configurations (% lagging catching up
with our DEPS). The variable use_custom_libcxx tells our build system to 
prefer the aforementioned copy to the system one.

Now, there are two reasons for using the /buildtools version of libcxx:
1) Sanitizers requires that the c++ library is built from sources, see
   https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo.
2) The libstdc++ situation is too wild on Linux. Modern debian distros are
   fine but Ubuntu Trusty still ships a libstdc++ that doesn't fully
   support C++11. Hence we enable this flag on Linux by default.
   We still retain libstdc++ coverage on the Travis bots by overriding
   use_custom_libcxx=false when we target a modern library (see the
   GCC7 target in .travis.yml).

Bug: 68256752
Change-Id: I70a99351cff6582663101bf26583f20460076013

build/BUILDCONFIG.gn

diff --git a/build/BUILDCONFIG.gn b/build/BUILDCONFIG.gn
index 1551cc6..52e75b4 100644
--- a/build/BUILDCONFIG.gn
+++ b/build/BUILDCONFIG.gn
@@ -19,13 +19,6 @@
 
 declare_args() {
   ar = "ar"
-  if (is_clang) {
-    cc = "clang"
-    cxx = "clang++"
-  } else {
-    cc = "gcc"
-    cxx = "g++"
-  }
 }
 
 # Platform detection
@@ -56,7 +49,7 @@
   "//build:extra_warnings",
   "//build:no_exceptions",
   "//build:no_rtti",
-  "//build:runtime_library",
+  "//build/libc++:config",
   "//build/sanitizers:sanitizers_cflags",
 ]
 
@@ -74,11 +67,14 @@
   deps = []
 }
 
+# Realistically the only shared_library that we build right now is libc++.so
+# when use_custom_libcxx=true (on Linux). Hence don't add a dependency on
+# libc++ itself on these targets.
 set_defaults("shared_library") {
   configs = default_configs
   configs += [ "//build:shared_library" ]
   deps = [
-    "//build:exe_and_shlib_deps",
+    "//build/sanitizers:deps",
   ]
 }
 
@@ -86,7 +82,8 @@
   configs = default_configs
   configs += [ "//build:executable" ]
   deps = [
-    "//build:exe_and_shlib_deps",
+    "//build/libc++:deps",
+    "//build/sanitizers:deps",
   ]
 }