parisc: Fix boot failure of 64-bit kernel

Commit c8921d72 ("parisc: Fix and improve kernel stack unwinding")
broke booting of 64-bit kernels. On 64-bit kernels function pointers are
actually function descriptors which require dereferencing. In this patch
we instead declare functions in assembly code which are referenced from
C-code as external data pointers with the ENTRY() macro and thus can use
a simple external reference to the functions.

Signed-off-by: Helge Deller <deller@gmx.de>
Fixes: c8921d72 ("parisc: Fix and improve kernel stack unwinding")