[BUG] unwinder symbols are no longer hidden with ndk-build

[DanAlbert]

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE := foo
LOCAL_SRC_FILES := foo.cpp
LOCAL_CPPFLAGS := -fexceptions -frtti
include $(BUILD_SHARED_LIBRARY)

APP_STL := c++_static

void foo() {
    throw 42;
}

$ ndk-build -C foo APP_ABI=arm64-v8a -B V=1
(build output shows -Wl,--exclude-libs,libgcc.a)
$ readelf -sW libs/arm64-v8a/libfoo.so| grep _Unwind
(shows global visibility unwind symbols)

Adding -Wl,--exclude-libs,libgcc_real.a to LOCAL_LDFLAGS fixes this, so this was caused by changing libgcc.a to be a version script (r19, if memory serves).

[DanAlbert]

This isn't as problematic as I'd feared. The ABI that this causes issues on (arm32) was unaffected for the same reason that it's the one that usually has problems: it uses a different unwinder. Since libunwind was always being linked first and --exclude-libs was still working fine with libunwind, the unwind symbols remained hidden in 32-bit arm outputs.

Still, I've fixed this for the other ABIs to defend against future ABI breaks: https://android-review.googlesource.com/c/platform/ndk/+/1133109