[llvm-readobj] Don't print out section names for STABS symbols

This diff is similar to what D71394 did for `llvm-objdump` -- it avoids
trying to look up a section name for STABS symbols, since some STABS
symbol types (like `N_OSO`) use the `n_sect` field to store other data
instead of a section index.

Differential Revision: https://reviews.llvm.org/D88468