runtime: use new CNT_MASK in lfstack

This is for SPARC64, a 64-bit processor that uses all 64-bits
of virtual addresses.  The idea is to use the low order 3 bits
to at least get a small ABA counter.  That should work since
pointers are aligned.  The idea is for SPARC64 to set CNT_MASK
== 7, PTR_BITS == 0, PTR_MASK == 0xffffffffffffff8.

Also add uintptr casts to avoid GCC warnings.  The gccgo
runtime code is compiled with GCC, and GCC warns when casting
between a pointer and a type of a different size.

R=dvyukov
CC=golang-dev
https://codereview.appspot.com/7225043