gdb: fix -Wtautological-overlap-compare warning in mips-linux-tdep.c

gdb: fix -Wtautological-overlap-compare warning in mips-linux-tdep.c

When building with clang 11, I get:

  CXX    mips-linux-tdep.o
/home/smarchi/src/binutils-gdb/gdb/mips-linux-tdep.c:643:30: error: overlapping comparisons always evaluate to true [-Werror,-Wtautological-overlap-compare]
      if (insn != 0x03e07821 || insn != 0x03e07825)
          ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/home/smarchi/src/binutils-gdb/gdb/mips-linux-tdep.c:636:30: error: overlapping comparisons always evaluate to true [-Werror,-Wtautological-overlap-compare]
      if (insn != 0x03e0782d || insn != 0x03e07825)
          ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

Indeed, given two different values, `insn` will always be different to
one of them, and these conditions always be true.

This code is meant to return if `insn` isn't one of these two values, so
the `||` should be replaced with `&&`.

gdb/ChangeLog:

	* mips-linux-tdep.c (mips_linux_in_dynsym_stub): Fix condition.