powerpc/dtrace: Fix fbt function probing for ELFv2

'.' function names exist only in ELFv1. ELFv2 does away with function
descriptors, and look more like they do on powerpc(32) and most other
platforms, as direct function pointers. Stop blacklisting regular function
names in ELFv2.

Submitted by: Brandon Bergren
Differential Revision: https://reviews.freebsd.org/D20346