parisc: add CONFIG_MLONGCALLS option to enable linkage of huge vmlinux executables

When building a 64bit kernel which includes all necessary drivers and
filesystems the vmlinux kernel often gets so huge, that the linker won't
be able to resolve the branch stubs. This patch overcomes this limit by
providing an option to compile the kernel with the -mlong-calls compiler
option.

Signed-off-by: Helge Deller <deller@gmx.de>