Make targetarch directory in build tree and copy arch/alpha files to it

Make targetarch directory in build tree and copy arch/alpha files to it
instead of using symlink.  The symlink broke scons's built-in include
dependency tacking.

Interestingly once it was fixed scons discovered two circular dependency
problems which are also fixed now.

SConscript:
    Make targetarch directory in build tree and copy arch/alpha files to it
    instead of using symlink.  The symlink broke scons's built-in include
    dependency tacking.
arch/alpha/ev5.hh:
    Get rid of circular #include dependence.
kern/kernel_stats.cc:
    Add needed header file.
kern/linux/linux_syscalls.hh:
kern/tru64/tru64_syscalls.hh:
    Replace targetarch/syscalls.hh with single template class declaration.

--HG--
extra : convert_revision : b8551623c1d441c6eb8d0651387e97e373128814