[ia64-linux] Allow libunwind to fetch register 0

[ia64-linux] Allow libunwind to fetch register 0

On ia64-linux, GDB sometimes prints the following error when trying
to switch to a different task:

    (gdb) task 3
    Register 0 is not available

This is a random failure that sometimes happens, sometimes does not.
The error comes from the fact that the libunwind library is requesting
the value of register 0 (zero): This eventually leads us to
ia64-linux-nat.c:ia64_linux_fetch_register.

This function relies on ia64_cannot_fetch_register to determine
whether or not we have access to the register's value.  The ptrace
interface does not provide the r0 value, and so we end up telling
the regcache that this register's value is not available.  And yet,
for r0, we do not need to ask ptrace for its value, since it is
always zero.

So, the fix was to add a special rule for supplying a nul value
when regnum == IA64_GR0_REGNUM.

gdb/ChangeLog:

        * ia64-linux-nat.c (ia64_linux_fetch_register): Add special
        handling for r0.