feature: print negative memory operand offsets as "-absvalue"

_From [timurrrr@google.com](https://code.google.com/u/timurrrr@google.com/) on June 11, 2013 13:28:35_

For this assembly (objdump -D):
c7 40 f8 00 00 00 00 movl $0x0,-0x8(%rax)
48 89 68 e8 mov %rbp,-0x18(%rax)
c6 04 28 00 movb $0x0,(%rax,%rbp,1)
eb c9 jmp

DR instrlist_disassemble produces this on Linux x64:
c7 40 f8 00 00 00 00 mov $0x00000000 -> 0xfffffff8(%rax)
48 89 68 e8 mov %rbp -> 0xffffffe8(%rax)
c6 04 28 00 mov $0x00 -> (%rax,%rbp,1)
eb c9 jmp

Please note that negative offsets are printed as 32-bits rather than 64-bits.
[Also I find them not very readable]

Is it a printing-only problem?

When I use "drutil_insert_get_mem_addr()" on such an operand and print the instrumentation with instrlist_disassemble it printst out the weird offset too:
48 8d 40 f8 lea 0xfffffff8(%rax) -> %rax

However, objdump thinks this is -0x8 again:
48 8d 40 f8 lea -0x8(%rax),%rax

This is confusing.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1186_