[libc++] accept ELF Tool Chain readelf format in tooling

ELF Tool Chain provides alternatives to most GNU binutils tools,
including readelf.  These tools are currently used by FreeBSD.

ELF Tool Chain's readelf currently emits headings for symbol table
information in a slightly different format compared to GNU or LLVM
readelf.  Accept both formats.

Reviewed by: philnik
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D140313