pch on sparc64

[David S. Miller]

PCH has been failing for quite some time on sparc64 and some other
similar Linux platforms.

The reason why is that file mmap()'s are enforced to certain boundaries
on sparc64 under Linux in order to address data-cache aliasing issues.

GCC wants the thing mapped at a specific address, and if it doesn't get
that specific address it fails.  Fixing this is not easy.

There were threads about this some time ago.  When PCH works on these
platforms, it is by chance.