Switch sparc64 to LLVM's libc++

Index: compiler/rustc_llvm/build.rs
--- compiler/rustc_llvm/build.rs.orig
+++ compiler/rustc_llvm/build.rs
@@ -476,9 +476,8 @@ fn main() {
     let llvm_static_stdcpp = tracked_env_var_os("LLVM_STATIC_STDCPP");
     let llvm_use_libcxx = tracked_env_var_os("LLVM_USE_LIBCXX");
 
-    let stdcppname = if target.contains("openbsd") {
-        if target.contains("sparc64") { "estdc++" } else { "c++" }
-    } else if target.contains("darwin")
+    let stdcppname = if target.contains("openbsd")
+        || target.contains("darwin")
         || target.contains("freebsd")
         || target.contains("windows-gnullvm")
         || target.contains("aix")
