Fill exception vectors with zero bytes

Fill exception vectors with zero bytes

The documentation of the GNU assembler specifies the following about
the .align assembler directive:
 "the padding bytes are normally zero. However, on some systems, if
 the section is marked as containing code and the fill value is
 omitted, the space is filled with no-op instructions."
(see https://sourceware.org/binutils/docs/as/Align.html)

When building Trusted Firmware, the AArch64 GNU assembler uses a
mix of zero bytes and no-op instructions as the padding bytes to
align exception vectors.

This patch mandates to use zero bytes to be stored in the padding
bytes in the exception vectors. In the AArch64 instruction set, no
valid instruction encodes as zero so this effectively inserts
illegal instructions. Should this code end up being executed for
any reason, it would crash immediately. This gives us an extra
protection against misbehaving code at no extra cost.

Change-Id: I4f2abb39d0320ca0f9d467fc5af0cb92ae297351