ArmVirtPkg/PrePi: Ensure timely execution of library constructors

PrePi has a bare metal entry point, and so it is in charge of calling
the library constructors once the C runtime has been initialized
sufficiently.

However, we are now relying on a HOB to have been constructed by the
time the MMU code runs, and so the constructors should be run before
that.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>