xtensa: fix target gdb build and 64-bit host side gdb

Fix build breakage, use the version of the ptrace header file in asm
instead of sys. Also, fix GDB running on 64 bit hosts. GDB was using
unsigned long for 32-bit registers, but unsigned long is 64 bit on
64-bit hosts.

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>