PR14524: support old embedded-locals notation on a per-function basis

PR14524: support old embedded-locals notation on a per-function basis

Adding the pragma /* unmangled */ to an embedded-C function now enables
the use of the pre-1.8 local names, i.e. THIS->n instead of STAP_ARG_n.
This aids migration of pre-1.8 scripts and tapsets by allowing mixing
two kinds of embedded functions (pre- and post-1.8) in the same code.