OLD_CANNOT_FETCH_REGISTER): New definitions for accessible registers
when accessing the registers via the U area.
(fetch_register, store_register): Use them.
(cannot_fetch_register, cannot_store_register): New functions,
all registers should be accessible if we have GETREGS support.
* config/i386/nm-linux.h: Use cannot_fetch/store_register for
CANNOT_FETCH/STORE_REGISTER definitions.