morello/disassembler: Fixed objdump of adrp inst with negative offset

morello/disassembler: Fixed objdump of adrp inst with negative offset

When an adrp instruction references a symbol that is more than a page
in memory behind the instruction, it would have a negative offset.
An example of this is:

foo:
        nop
        .zero 4096
        adrp c0, foo

where adrp references 'foo' that is more than a page in memory behind
it.

In the case where the offset is negative, when translating from its
binary format, the offset was seen as an unsigned integer, which
caused a spurious high bit set in the resolved address in the adrp
instruction like in:

0000000000400078 <foo>:
 400078:   d503201f   nop
        ...
 40107c:   f0ffffe0   adrp    c0, 100400000 <__bss_end__+0xfffeeff8>

There was an issue with how the imm field of the adrp instruction was
extracted in `aarch64_ext_imm`. The value was not sign extended
correctly for capability mode targets. This was caused by the imm
field having its `P` bit being removed before the sign extension,
which is exclusive to 64-bit capability targets. This was remedied
by shortening the width of the imm field before sign extending the
imm value, resulting in:

0000000000400078 <foo>:
 400078:   d503201f   nop
        ...
 40107c:   f0ffffe0   adrp    c0, 400000 <foo-0x78>