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

The buildtools roll includes these revisions:
https://chromium.googlesource.com/chromium/buildtools/+log/437a616be5b2056336ed3ca5c33c41f7788ede09..a2cbf26d9ed33836bc3e412b4dc0df259e4a1525

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-Commit-Position: refs/heads/master@{#532259}

DEPS

@@ -95,7 +95,7 @@ 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': '437a616be5b2056336ed3ca5c33c41f7788ede09',
+  'buildtools_revision': 'f115f4786771afdfacb18a2299541b23e21f4792',
   # 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/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"

build/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++" ]

build/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",