Remove libunwind arm dependency from libc++abi (src-side changes)

The buildtools roll includes these revisions:
437a616be5..a2cbf26d9e

BUG=803679
R=thakis@chromium.org

Change-Id: Ie398be7d8ef52d6a733f52043300449b3d35e947
Reviewed-on: https://chromium-review.googlesource.com/875330
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#532259}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ea4edeec4194c6e751d3be2d5fa262a2475f6c47

config/c++/c++.gni

@@ -39,4 +39,3 @@ use_custom_libcxx =
 
 libcxx_prefix = "//buildtools/third_party/libc++/trunk"
 libcxxabi_prefix = "//buildtools/third_party/libc++abi/trunk"
-libunwind_prefix = "//buildtools/third_party/libunwind/trunk"

config/posix/BUILD.gn

@@ -49,14 +49,6 @@ config("runtime_library") {
       "-isystem" + rebase_path("$libcxxabi_prefix/include", root_build_dir),
     ]
 
-    # This condition should match the one in buildtools/third_party/libc++abi.
-    if ((is_linux && current_cpu == "arm") || is_fuchsia) {
-      cflags_c += [ "-isystem" +
-                    rebase_path("$libunwind_prefix/include", root_build_dir) ]
-      cflags_cc += [ "-isystem" +
-                     rebase_path("$libunwind_prefix/include", root_build_dir) ]
-    }
-
     # Make sure we don't link against libc++ or libstdc++.
     if (is_clang) {
       ldflags += [ "-nostdlib++" ]

nocompile.gni

@@ -86,7 +86,6 @@ if (enable_nocompile_tests) {
         "-nostdinc++",
         "-isystem" + rebase_path("$libcxx_prefix/include", root_build_dir),
         "-isystem" + rebase_path("$libcxxabi_prefix/include", root_build_dir),
-        "-isystem" + rebase_path("$libunwind_prefix/include", root_build_dir),
         "-std=c++14",
         "-Wall",
         "-Werror",