arm: relocate the exception vectors

This commit relocates the exception vectors.
As ARM1176 and ARMv7 have the security extensions, it uses VBAR.  For
the other ARM processors, it copies the relocated exception vectors to
the correct address: 0x00000000 or 0xFFFF0000.

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Tom Warren <twarren@nvidia.com>