This patch adds support for Armv8-R AArch64.
Armv8-R AArch64 has no EL3, so we don't set vbar_el3, and adjust the
code to set up the MPU for Armv8-R. So build a different flavour of the
startup code to support that.
We also add a specs file that uses this alternative startup code which
can be used with Armv8-R AArch64 models.
Alex Coplan via Newlib