Expose unwinder APIs in NDK stubs for R and up.

These APIs are exposed in the on-device libc.so's .dynsym table from R
and up (e.g. _Unwind_xxx@@LIBC_R), but they were only available in the
APEX and LLNDK stubs. Expose the symbols from the NDK stubs too so that
the LLVM toolchain build can build a libc++.so that imports the
unwinder from libc.so. (The platform/APEX libc++.so will become a
toolchain prebuilt.)

Eventually this change will also allows apps to use the unwinder from
libc.so rather than linking libunwind.a statically.

Bug: http://b/175635923
Test: treehugger
Change-Id: I7ba9cef9a4727b49dd717e25a0321bf2889694de
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 5695dc6..4a4e607 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1518,34 +1518,34 @@
     tss_set;
 
     # Unwinder implementation
-    __aeabi_unwind_cpp_pr0; # apex llndk arm
-    __aeabi_unwind_cpp_pr1; # apex llndk arm
-    __aeabi_unwind_cpp_pr2; # apex llndk arm
-    __deregister_frame; # apex llndk arm64 x86 x86_64
-    __gnu_unwind_frame; # apex llndk arm
-    __register_frame; # apex llndk arm64 x86 x86_64
-    _Unwind_Backtrace; # apex llndk
-    _Unwind_Complete; # apex llndk arm
-    _Unwind_DeleteException; # apex llndk
-    _Unwind_Find_FDE; # apex llndk
-    _Unwind_FindEnclosingFunction; # apex llndk
-    _Unwind_ForcedUnwind; # apex llndk arm64 x86 x86_64
-    _Unwind_GetCFA; # apex llndk
-    _Unwind_GetDataRelBase; # apex llndk
-    _Unwind_GetGR; # apex llndk
-    _Unwind_GetIP; # apex llndk
-    _Unwind_GetIPInfo; # apex llndk
-    _Unwind_GetLanguageSpecificData; # apex llndk
-    _Unwind_GetRegionStart; # apex llndk
-    _Unwind_GetTextRelBase; # apex llndk
-    _Unwind_RaiseException; # apex llndk
-    _Unwind_Resume; # apex llndk
-    _Unwind_Resume_or_Rethrow; # apex llndk
-    _Unwind_SetGR; # apex llndk
-    _Unwind_SetIP; # apex llndk
-    _Unwind_VRS_Get; # apex llndk arm
-    _Unwind_VRS_Pop; # apex llndk arm
-    _Unwind_VRS_Set; # apex llndk arm
+    __aeabi_unwind_cpp_pr0; # arm
+    __aeabi_unwind_cpp_pr1; # arm
+    __aeabi_unwind_cpp_pr2; # arm
+    __deregister_frame; # arm64 x86 x86_64
+    __gnu_unwind_frame; # arm
+    __register_frame; # arm64 x86 x86_64
+    _Unwind_Backtrace;
+    _Unwind_Complete; # arm
+    _Unwind_DeleteException;
+    _Unwind_Find_FDE;
+    _Unwind_FindEnclosingFunction;
+    _Unwind_ForcedUnwind; # arm64 x86 x86_64
+    _Unwind_GetCFA;
+    _Unwind_GetDataRelBase;
+    _Unwind_GetGR;
+    _Unwind_GetIP;
+    _Unwind_GetIPInfo;
+    _Unwind_GetLanguageSpecificData;
+    _Unwind_GetRegionStart;
+    _Unwind_GetTextRelBase;
+    _Unwind_RaiseException;
+    _Unwind_Resume;
+    _Unwind_Resume_or_Rethrow;
+    _Unwind_SetGR;
+    _Unwind_SetIP;
+    _Unwind_VRS_Get; # arm
+    _Unwind_VRS_Pop; # arm
+    _Unwind_VRS_Set; # arm
 } LIBC_Q;
 
 LIBC_S { # introduced=S