Build llvm-amdgpu's libcxx with cmake --build (#31403)

CMAKE_GENERATOR may be set to something other than "Unix Makefiles" in
which case the build would fail because there is no Makefile for make to
build.

var/spack/repos/builtin/packages/llvm-amdgpu/package.py

@@ -202,4 +202,4 @@ def post_install(self):
                 ]
                 cmake_args.extend(self.cmake_args())
                 cmake(*cmake_args)
-                make()
+                cmake("--build", ".")