Fix -Wpointer-sign errors in ia64-tdep.c

gdb/ChangeLog:

        * ia64-tdep.c (ia64_access_fpreg): Change cast of val
        from "char *" to "gdb_byte *".
        (ia64_access_rse_fpreg, ia64_access_mem): Likewise.
Fix -Wpointer-sign errors in solib-ia64-hpux.c

gdb/ChangeLog:

        * solib-ia64-hpux.c (ia64_hpux_read_dynamic_info): Change cast
        of &info->load_map from "char *" to "gdb_byte *".
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 *".