This patch fixes issues with signed zero offsets in the arm assembler and 
disassembler.

The ARM ISA encodes offsets in sign/magnitude representation, so there are two 
representations of zero.  The ISA specifies how the assembler should select the 
right one, and the disassembler should explicitly show -0.  For +0, it should 
elide the offset in certain circumstances.

This patch fixes up things so that, for instance
	ldr  r0,[r1,#-0]
is encoded correctly.

tested on arm-eabi, ok?

nathan
-- 
Nathan Sidwell