Switch libc.so and linker to prebuilt LLVM libunwind

For libc.so, use a special build of libunwind.a whose symbols aren't
hidden ("libunwind-exported"), because libc.so exports the _Unwind_*
APIs.

Bug: http://b/153025717
Test: bionic unit tests
Change-Id: I7435e076ec8cc4410e3e6086d3cf5d2c6011c80c
diff --git a/linker/Android.bp b/linker/Android.bp
index 3ebae59..efda312 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -232,19 +232,15 @@
     arch: {
         arm: {
             srcs: [":linker_sources_arm"],
-            static_libs: ["libunwind_llvm"],
         },
         arm64: {
             srcs: [":linker_sources_arm64"],
-            static_libs: ["libgcc_stripped"],
         },
         x86: {
             srcs: [":linker_sources_x86"],
-            static_libs: ["libgcc_stripped"],
         },
         x86_64: {
             srcs: [":linker_sources_x86_64"],
-            static_libs: ["libgcc_stripped"],
         },
     },
 
@@ -292,6 +288,7 @@
         "libc_nomalloc",
         "libc_dynamic_dispatch",
         "libm",
+        "libunwind",
     ],
 
     // Ensure that if the linker needs __gnu_Unwind_Find_exidx, then the linker will have a