Switch platform EH unwinder to prebuilt LLVM libunwind

For the platform libc++ STL, remove the
-Wl,--exclude-libs,libunwind_llvm.a argument, which is redundant with
the same argument in deviceGlobalLdflags.

Bug: http://b/153025717
Test: device boots
Change-Id: Idd7791d52f74aab2d5f59419fb75f841fc29a2eb
diff --git a/apex/allowed_deps.txt b/apex/allowed_deps.txt
index 57b18de..f59408e 100644
--- a/apex/allowed_deps.txt
+++ b/apex/allowed_deps.txt
@@ -564,6 +564,7 @@
 prebuilt_libclang_rt.builtins-arm-android(minSdkVersion:(no version))
 prebuilt_libclang_rt.builtins-i686-android(minSdkVersion:(no version))
 prebuilt_libclang_rt.builtins-x86_64-android(minSdkVersion:(no version))
+prebuilt_libunwind(minSdkVersion:(no version))
 prebuilt_test_framework-sdkextensions(minSdkVersion:(no version))
 server_configurable_flags(minSdkVersion:29)
 service-permission(minSdkVersion:current)