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: NStefan Hajnoczi <stefanha@redhat.com>

disas/arm.c

@@ -1779,7 +1779,7 @@ print_insn_coprocessor (bfd_vma pc, struct disassemble_info *info, long given,
 
 			/* Is ``imm'' a negative number?  */
 			if (imm & 0x40)
-			  imm |= (-1 << 7);
+			  imm |= (~0u << 7);
 
 			func (stream, "%d", imm);
 		      }