loader: move x64-specific stuff from premain

move the arch-specific stuff in premain to
arch/x64/arch-setup.cc.

Introduce arch_init_premain() and arch_setup_tls().

arch_init_premain() is supposed to perform arch-specific
initialization before the common premain code is run.

arch_setup_tls() is run _after_ the common setup_tls code.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>