fix dlsym lookup of function symbols on fdpic

fix dlsym lookup of function symbols on fdpic

previously these resolved to the code address rather than the address
of the function descriptor.

the conditions for accepting or rejecting symbols are quite
inconsistent between the different points in the dynamic linker code
where such decisions are made. this commit attempts to be at least as
correct as anything already there, but does not improve consistency.
it has been tested to correctly avoid symbols that are merely
references to functions defined in other modules, at least in simple
usage, but at some point all symbol lookup logic should be reviewed
and refactored/unified.