fbt/x86: Extract arg1 for return probes from the trapframe

dtrace invop handlers have access to the whole trapframe, just use that
to extract %rax/%eax for return probes instead of relying on an
additional parameter to the handler. No functional change intended.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit a7aa3d4d758dc60894e7f9c99411a64837fd3c18)