arm64: vdso: move to _install_special_mapping and remove arch_vma_name

_install_special_mapping replaces install_special_mapping and removes
the need to detect special VMA in arch_vma_name.

This patch moves the vdso and compat vectors page code over to the new
API.

Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>