Three updates to PowerPC FBT:

* Use a constant to define the number of stack frames in a probe exception.
* Only allow function symbols in powerpc64 ('.' prefixed)
* Set the fbtp_roffset for return probes, so the correct dtrace_probe call is
  made.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273570 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Whitespace

X-MFC-with:	r273570
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273571 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f