gdb: fix -Wtautological-overlap-compare error in h8300-tdep.c

gdb: fix -Wtautological-overlap-compare error in h8300-tdep.c

Compiling with clang 11 gives us:

      CXX    h8300-tdep.o
    /home/smarchi/src/binutils-gdb/gdb/h8300-tdep.c:225:21: error: overlapping comparisons always evaluate to false [-Werror,-Wtautological-overlap-compare]
                  if (disp < 0 && disp > 0xffffff)
                      ~~~~~~~~~^~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/h8300-tdep.c:203:17: error: overlapping comparisons always evaluate to false [-Werror,-Wtautological-overlap-compare]
              if (disp < 0 && disp > 0xffffff)
                  ~~~~~~~~~^~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/h8300-tdep.c:184:17: error: overlapping comparisons always evaluate to false [-Werror,-Wtautological-overlap-compare]
              if (disp < 0 && disp > 0xffffff)
                  ~~~~~~~~~^~~~~~~~~~~~~~~~~~

Indeed, disp (of type LONGEST) can't be less than 0 and greater than
0xffffff.

Fix it by changing the way we check if disp is negative.  Check the sign
bit of disp, which is a 24-bit number.

gdb/ChangeLog:

	* h8300-tdep.c (h8300_is_argument_spill): Change how we check
	whether disp is negative.