arm: boot: Support big-endian elfs

arm: boot: Support big-endian elfs

Support ARM big-endian ELF files in system-mode emulation. When loading
an elf, determine the endianness mode expected by the elf, and set the
relevant CPU state accordingly.

With this, big-endian modes are now fully supported via system-mode LE,
so there is no need to restrict the elf loading to the TARGET
endianness so the ifdeffery on TARGET_WORDS_BIGENDIAN goes away.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: fix typo in comments]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>