arm-bsp/arch-armv8r64: Introduce armv8r64 compiler flags

Introduce the proper compiler flags for armv8r64, and also
disable -march=armv8-r for boot-wrapper-aarch64

Issue-Id: SCM-2342
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I3d2fac232139b057a5d1a3a8c6f5b825f95d709a
Signed-off-by: Jon Mason <jon.mason@arm.com>