amd64 pcpu: fix clobbers, suppress warnings, and clean up

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.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45827