[gn build] Remove --rtlib=libgcc for Android builds

Recent Android NDKs don't ship with libgcc

Author: aeubanks

llvm/utils/gn/build/toolchain/target_flags.gni

@@ -15,9 +15,6 @@ if (current_os == "android") {
     "--sysroot=$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64/sysroot",
     "--gcc-toolchain=$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64",
   ]
-  if (current_cpu != "arm") {
-    target_flags += [ "-rtlib=libgcc" ]
-  }
   target_ldflags += [ "-static-libstdc++" ]
   if (current_cpu == "arm") {
     target_flags += [ "-march=armv7-a" ]