x86/vdso: Move VDSO image init to vdso2c generated code

Generate an init function for each VDSO image, replacing init_vdso() and
sysenter_setup().

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230124184019.26850-1-brgerst@gmail.com