Fix various -Wpointer-sign warnings in darwin-nat.c

gdb/ChangeLog:

        * darwin-nat.c (darwin_read_write_inferior): Change types
        of parameters rdaddr and wraddr to "gdb_byte *". Change type
        of copy_count to "mach_msg_type_number_t".
        (darwin_read_dyld_info): Change type of parameter
        rdaddr to "gdb_byte *".