[IA64] Fix convert_to_non_syscall() so gdb inferior calls work again

Fix convert_to_non_syscall() so it arranges for the kernel to be left
via ia64_leave_kernel() rather than ia64_leave_syscall().  The latter
no longer tolerates being called with pSys=0 and pNonSys=1.

Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>