xen/arm64: head: Introduce a macro to get a PC-relative address of a symbol

xen/arm64: head: Introduce a macro to get a PC-relative address of a symbol

Arm64 provides instructions to load a PC-relative address, but with some
limitations:
   - adr is enable to cope with +/-1MB
   - adrp is enale to cope with +/-4GB but relative to a 4KB page
     address

Because of that, the code requires to use 2 instructions to load any Xen
symbol. To make the code more obvious, introducing a new macro adr_l is
introduced.

The new macro is used to replace a couple of open-coded use in
efi_xen_start.

The macro is copied from Linux 5.2-rc4.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>