[arm] fix assembler crash

[Nathan Sidwell]

This patch fixes a bug where the assembler seg faults if you try and use the 
thumb-only cbnz or cbz instructions.  It also augments the error messages 
emitted when you use a thumb- or arm-only instruction in the wrong mode.  Rather 
than claim the cpu doesn't support that instruction, it now says it doesn't 
support it *in $FOO mode* (where FOO is Thumb or ARM as appropriate).

Tested on arm-eabi, ok?

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery

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