handle variables stored in muliple consecutive registers

handle variables stored in muliple consecutive registers

gdb/ChangeLog:

        * value.h (read_frame_register_value): Add declaration.
        * findvar.c (read_frame_register_value): New function.
        (value_from_register): Use read_frame_register_value
        instead of get_frame_register_value + value_contents_copy
        to get value contents.