i386 pcpu: fix clobbers, suppress warnings, and clean up

• 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.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45826