Compute the correct symbol size in llvm-nm even without --print-size

In llvm-nm, the symbol size was being computed only with --print-size option,
even though it was being printed in other cases, such as with --format=posix.

This patch simply removes the guard, so that the size is computed
independently of the later decision to print it or not.

Fixes PR39997.

Differential Revision: https://reviews.llvm.org/D57599

llvm-svn: 353011