ELF: Do less if HAVE_CXXABI_H is not defined.

If HAVE_CXXABI_H is not defined, this function is the identity function.
Because HAVE_CXXABI_H did not protect the entire function, it did
extra stuffs before returning the argument.

The new code calls fewer functions. This should help developers understand
this piece of code.

llvm-svn: 233460