[sanitizers] Get the proper symbol version when long double transition is involved.

On linux, some architectures had an ABI transition from 64-bit long double
(ie. same as double) to 128-bit long double.  On those, glibc symbols
involving long doubles come in two versions, and we need to pass the
correct one to dlvsym when intercepting them.

A few more functions we intercept are also versioned (all printf, scanf,
strtold variants), but there's no need to fix these, as the REAL() versions
are never called.

Differential Revision: http://reviews.llvm.org/D19555

llvm-svn: 267794