Support ppc64le and fix cross architecture bootstrapping

This pull request adds support for ppc64le in pmbootstrap and fixes bootstrapping of a new architecture. The generated cross-compiler still needs the same fixups as pmaports@ad9a7874, but works fine and the QEMU target can be built successfully and booted (pending pmaports pull request with necessary changes, will open after this gets merged).

pmb/core/test_arch.py

@@ -27,6 +27,7 @@ def test_valid_arches():
     assert Arch.aarch64 in Arch.supported()
     assert Arch.armhf in Arch.supported()
     assert Arch.armv7 in Arch.supported()
+    assert Arch.riscv64 in Arch.supported()
 
     # kernel arch
     assert Arch.x86.kernel() == "x86"