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

cc/config/global.go

@@ -90,9 +90,12 @@ var (
 		"-Wl,--warn-shared-textrel",
 		"-Wl,--fatal-warnings",
 		"-Wl,--no-undefined-version",
+		// TODO: Eventually we should link against a libunwind.a with hidden symbols, and then these
+		// --exclude-libs arguments can be removed.
 		"-Wl,--exclude-libs,libgcc.a",
 		"-Wl,--exclude-libs,libgcc_stripped.a",
 		"-Wl,--exclude-libs,libunwind_llvm.a",
+		"-Wl,--exclude-libs,libunwind.a",
 	}
 
 	deviceGlobalLldflags = append(ClangFilterUnknownLldflags(deviceGlobalLdflags),

cc/stl.go

@@ -140,11 +140,7 @@ func needsLibAndroidSupport(ctx BaseModuleContext) bool {
 }
 
 func staticUnwinder(ctx android.BaseModuleContext) string {
-	if ctx.Arch().ArchType == android.Arm {
-		return "libunwind_llvm"
-	} else {
-		return "libgcc_stripped"
-	}
+	return "libunwind"
 }
 
 func (stl *stl) deps(ctx BaseModuleContext, deps Deps) Deps {
@@ -192,6 +188,7 @@ func (stl *stl) deps(ctx BaseModuleContext, deps Deps) Deps {
 		if needsLibAndroidSupport(ctx) {
 			deps.StaticLibs = append(deps.StaticLibs, "ndk_libandroid_support")
 		}
+		// TODO: Switch the NDK over to the LLVM unwinder for non-arm32 architectures.
 		if ctx.Arch().ArchType == android.Arm {
 			deps.StaticLibs = append(deps.StaticLibs, "ndk_libunwind")
 		} else {
@@ -231,10 +228,6 @@ func (stl *stl) flags(ctx ModuleContext, flags Flags) Flags {
 					// Use Win32 threads in libc++.
 					"-D_LIBCPP_HAS_THREAD_API_WIN32")
 			}
-		} else {
-			if ctx.Arch().ArchType == android.Arm {
-				flags.Local.LdFlags = append(flags.Local.LdFlags, "-Wl,--exclude-libs,libunwind_llvm.a")
-			}
 		}
 	case "libstdc++":
 		// Nothing

cc/testing.go

@@ -109,6 +109,16 @@ func GatherRequiredDepsForTest(oses ...android.OsType) string {
 			src: "",
 		}
 
+		toolchain_library {
+			name: "libunwind",
+			defaults: ["linux_bionic_supported"],
+			vendor_available: true,
+			product_available: true,
+			recovery_available: true,
+			native_bridge_supported: true,
+			src: "",
+		}
+
 		toolchain_library {
 			name: "libclang_rt.fuzzer-arm-android",
 			vendor_available: true,