disas/arm: avoid clang shifting negative signed warning

clang 3.7.0 on x86_64 warns about the following:

  disas/arm.c:1782:17: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
    imm |= (-1 << 7);
            ~~ ^

Note that this patch preserves the tab indent in this source file
because the surrounding code still uses tabs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
