std.zig.target: Consider libsupc++ to be a libc++ library name.

This is, roughly, GCC's equivalent of libc++abi.

lib/std/zig/target.zig

@@ -263,7 +263,8 @@ pub fn isLibCxxLibName(target: std.Target, name: []const u8) bool {
 
     return eqlIgnoreCase(ignore_case, name, "c++") or
         eqlIgnoreCase(ignore_case, name, "stdc++") or
-        eqlIgnoreCase(ignore_case, name, "c++abi");
+        eqlIgnoreCase(ignore_case, name, "c++abi") or
+        eqlIgnoreCase(ignore_case, name, "supc++");
 }
 
 fn eqlIgnoreCase(ignore_case: bool, a: []const u8, b: []const u8) bool {