[arm] Clean up generation of BE8 format images.

[arm] Clean up generation of BE8 format images.

The existing code in arm/bpabi.h was quite fragile and relied on matching
specific CPU and/or architecture names.  The introduction of the option
format for -mcpu and -march broke that in a way that would be non-trivial
to fix by updating the list.  The hook in that file was always a pain
as it required every new CPU being added to be add an update here as well
(easy to miss).

I've fixed that problem once and for all by adding a new callback into
the driver to select the correct BE8 behaviour.  This uses features in
the ISA capabilities list to select whether or not to use BE8 format
during linking.

I also noticed that if the user happened to pass both -mbig-endian and
-mlittle-endian on the command line then the linker spec rules would
get somewhat confused and potentially do the wrong thing.  I've fixed that
by marking these options as opposites in the option descriptions.  The
driver will now automatically suppress overridden options leading to the
correct desired behavior.

Whilst fixing this I noticed a couple of anomolus cases in the
existing BE8 support: we were not generating BE8 format for ARMv6 or
ARMv7-R targets.  While the ARMv6 status was probably deliberate at
the time, this is probably not a good idea in the long term as the
alternative, BE32, has been deprecated by ARM.  After discussion with
a couple of colleagues I've decided to change this, but to then add an
option to restore the existing behaviour at the user's option.  So
this patch introduces two new options (opposites) -mbe8 and -mbe32.

This is a quiet behavior change, so I'll add a comment to the release
notes shortly.

	* common/config/arm/arm-common.c (arm_be8_option): New function.
	* config/arm/arm-isa.h (isa_feature): Add new feature bit isa_bit_be8.
	(ISA_ARMv6): Add isa_bit_be8.
	* config/arm/arm.h (arm_be8_option): Add prototype.
	(BE8_SPEC_FUNCTION): New define.
	(EXTRA_SPEC_FUNCTIONS): Add BE8_SPEC_FUNCTION.
	* config/arm/arm.opt (mbig-endian): Mark as Negative of mlittle-endian.
	(mlittle-endian): Similarly.
	(mbe8, mbe32): New options.
	* config/arm/bpabi.h (BE8_LINK_SPEC): Call arm_be8_option.
	* doc/invoke.texi (ARM Options): Document -mbe8 and -mbe32.

From-SVN: r249909