std.Target: Move isLib{C,Cxx}LibName() to std.zig.target.

These are really answering questions about the Zig compiler's capacity to
provide a libc/libc++ implementation. As such, std.zig.target seems like a more
fitting place for these.

lib/std/Build/Module.zig

@@ -492,11 +492,11 @@ pub fn linkSystemLibrary(
     const b = m.owner;
 
     const target = m.requireKnownTarget();
-    if (target.isLibCLibName(name)) {
+    if (std.zig.target.isLibCLibName(target, name)) {
         m.link_libc = true;
         return;
     }
-    if (target.isLibCxxLibName(name)) {
+    if (std.zig.target.isLibCxxLibName(target, name)) {
         m.link_libcpp = true;
         return;
     }