arm64: elf: vDSO code page discovery

Like in normal vDSOs, when compat vDSOs are enabled the auxiliary
vector symbol AT_SYSINFO_EHDR needs to point at the address of the
vDSO code, to allow the dynamic linker to find it.

This patch adds the necessary code to the elf arm64 module to make this
possible.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>