These changes mostly apply to the !__SEG_GS section, which is no longer
the normal compilation path. They're made to be consistent with changes
to i386.
- Add missing cc clobber to __PCPU_ADD (which is currently unused).
- Allow the compiler the opportunity to marginally improve code generation from __PCPU_PTR by letting it figure out how to do the add (also removing the addition fixes a missing cc clobber).
- Quiet gcc -Warray-bounds by using constant operands instead of bogus memory references.
- Remove the struct s s temporaries, just cast through the type.