[m68k] fix return value extraction

[Nathan Sidwell]

I discovered gdb's idea of how values are returned differed significantly from 
how gcc actually returned things.  Although gcc might be at variance from the 
ABI specification, I didn't want to change gcc's behaviour and thereby introduce 
ABI incompatibilities there.  AFAICT gcc's behaviour has been unchanged for a while.

*) GCC passes smalls structs that have an integral mode in a register, not small 
structs per se.  It is annoying that an implementation detail of gcc's internals 
is affecting the ABI this way.  I've reproduced what I think is an equivalent 
algorithm, so gdb can determine this.

*) GCC never sets %a0 to point to a returned structure.  Neither does it return 
pointer values in %a0.

*) coldfire chips really need the stack to be aligned to a 4 byte boundary.

*) In order for bare metal elf to work, I've defaulted gdb's configuration to be 
that, and added an os initialization routine for a.out.

*) I've enabled the linux sniffers for a uclinux target.

tested on a coldfire target board bare metal.

ok?

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

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