efi/arm64: libstub: avoid SetVirtualAddressMap() when possible

EFI's SetVirtualAddressMap() runtime service is a horrid hack that we'd
like to avoid using, if possible. For 64-bit architectures such as
arm64, the user and kernel mappings are entirely disjoint, and given
that we use the user region for mapping the UEFI runtime regions when
running under the OS, we don't rely on SetVirtualAddressMap() in the
conventional way, i.e., to permit kernel mappings of the OS to coexist
with kernel region mappings of the firmware regions. This means that, in
principle, we should be able to avoid SetVirtualAddressMap() altogether,
and simply use the 1:1 mapping that UEFI uses at boot time. (Note that
omitting SetVirtualAddressMap() is explicitly permitted by the UEFI
spec).

However, there is a corner case on arm64, which, if configured for
3-level paging (or 2-level paging when using 64k pages), may not be able
to cover the entire range of firmware mappings (which might contain both
memory and MMIO peripheral mappings).

So let's avoid SetVirtualAddressMap() on arm64, but only if the VA space
is guaranteed to be of sufficient size.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>