Disable export_libcxxabi_from_executables when building on iOS.

-Wl,-undefined,dynamic_lookup is deprecated on iOS Simulator, which
prevents us from linking libc++abi into the main executable and
exporting symbols for shared libraries to use. Instead, disallow
export_libcxxabi_from_executables on iOS, since it is effectively
unsupported.

BUG=1095852

Change-Id: I35204f9ccf668941f9e058aa05ab3caa9eaf376b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250218Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779959}

build/config/c++/c++.gni

@@ -58,7 +58,7 @@ declare_args() {
 # libc++abi needs to be exported from executables to be picked up by shared
 # libraries on certain instrumented builds.
 export_libcxxabi_from_executables =
-    use_custom_libcxx && !is_win && !is_component_build &&
+    use_custom_libcxx && !is_ios && !is_win && !is_component_build &&
     (is_asan || is_ubsan_vptr)
 
 # On Android, many shared libraries get loaded from the context of a JRE.  In