backtrace: Introduce backtrace function

This function diplays the backtrace, the current EL and security state
to allow a post-processing tool to choose the right binary to interpret
the dump.

The output can be fed to GNU addr2line to resolve function names given
an ELF binary compiled with debug information. The "-i" flag is
recommended to improve display in case of inlined functions. The *.dump
files generated during the build process can also be used.

The function works in AArch64 and AArch32. In AArch32 it only works in
A32 mode (without T32 interworking), which is enforced in the Makefile.

Sample output of a backtrace at EL3:

    BACKTRACE: START: function_name
    0: EL3: 0x798
    1: EL3: 0x538
    2: EL3: 0x550
    3: EL3: 0x55c
    4: EL3: 0x568
    5: EL3: 0x5a8
    6: EL3: 0xf4
    BACKTRACE: END: function_name

In order to enable it the new option ENABLE_BACKTRACE must be set to 1.
This option is set to 1 by default only in AArch64 debug builds. As
usual, it can be overridden by the platform makefile and in the build
command line.

Change-Id: Icaff39b0e5188329728be2f3c72b868b2368e794
Co-authored-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>