Fix Android ASAN build

Android+ASAN was already using libc++, so don't confuse it with the host
library.

Bug: skia:
Change-Id: Ibdeb40007f9d346a84adb39de240299049c0c55e
Reviewed-on: https://skia-review.googlesource.com/107721
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 7c35f71..c737dd9 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -249,6 +249,8 @@
     ]
     if (!is_win) {
       cflags += [ "-fno-omit-frame-pointer" ]
+    }
+    if (is_linux) {
       cflags_cc += [ "-stdlib=libc++" ]
       ldflags += [ "-stdlib=libc++" ]
     }