arm: initialize gd for AArch64

Commit adc421e4 "arm: move gd handling outside of C code" removed
the call to arch_setup_gd() on ARM and replaced it with assembly code
in crt0.S. However, AArch64 uses a different startup file, and the same
change was not made to it. This leaves gd uninitialized on AArch64, which
typically leads to hangs or crashes. This change fixes that.

Fixes: adc421e4 ("arm: move gd handling outside of C code")
Signed-off-by: Stephen Warren <swarren@nvidia.com>