arm: poison initmem when it is freed.

arm: poison initmem when it is freed.

The current byte sequence is '0xcc' which makes sense on x86,
but on ARM it is:

cccccccc        stclgt  12, cr12, [ip], {204}   ; 0xcc

Picking something more ARM applicable such as:

efefefef        svc     0x00efefef

Creates a nice crash if one executes that code:
(XEN) CPU1: Unexpected Trap: Supervisor Call

But unfortunately that may not be a good choice either as in the future
we may want to implement support for it.

Julien suggested that we use a 4-byte insn instruction instead
of trying to work with one byte. To make sure nothing goes bad
we also require that the __init_[begin|end] be aligned properly.

As such on ARM 32 we use the udf instruction (see A8.8.247
in ARM DDI 0406C.c) and on ARM 64 use the AARCH64_BREAK_FAULT
instruction (aka brk instruction).

We don't have to worry about Thumb code so this instruction
is a safe to execute.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>