* gdbcore.h (struct regcache): Add forward declaration.
(struct core_fns): Add REGCACHE argument to core_read_registers
callback.
* corelow.c (get_core_register_section): Add REGCACHE argument,
use it instead of current_regcache, pass it to core_read_registers
callback.
(get_core_registers): Add current_regcache as parameter to
get_core_register_section calls.