Add libunwind to libc_musl.so

Embed libunwind in libc_musl.so to provide the _Unwind* symbols,
which allows removing the dependency on libgcc.

Bug: 215802826
Test: m USE_HOST_MUSL=true host-native
Change-Id: I786354ba916e5cb568ea291a31b197b52f0cbfa7
diff --git a/Android.bp b/Android.bp
index 51bd202..2404954 100644
--- a/Android.bp
+++ b/Android.bp
@@ -235,6 +235,7 @@
         "libc_musl_nossp",
         "libexecinfo",
         "libb64",
+        "libunwind",
     ],
     multilib: {
         lib32: {