gdb: fix nr_bits gdb_assert in append_flags_type_field

gdb: fix nr_bits gdb_assert in append_flags_type_field

The assertion

    gdb_assert (nr_bits >= 1 && nr_bits <= type_bitsize);

is not correct.  Well, it's correct in that we do want the number of
bits to be in the range [1, type_bitsize].  But we don't check anywhere
that the end of the specified flag is within the containing type.

The following code should generate a failed assertion, as the flag goes
past the 32 bits of the underlying type, but it's currently not caught:

    static void
    test_print_flag (gdbarch *arch)
    {
      type *flags_type = arch_flags_type (arch, "test_type", 32);
      type *field_type = builtin_type (arch)->builtin_uint32;
      append_flags_type_field (flags_type, 31, 2, field_type, "invalid");
    }

(You can test this by registering it as a selftest using
selftests::register_test_foreach_arc and running.)

Change the assertion to verify that the end bit is within the range of
the underlying type.  This implicitly verifies that nr_bits is not
too big as well, so we don't need a separate assertion for that.

Change-Id: I9be79e5fd7a5917bf25b03b598727e6274c892e8
Co-Authored-By: Tony Tye <Tony.Tye@amd.com>