nptl_db: Re-use the ELF-to-abilist converter for ABI checking

The previous approach uses readelf -DWs, which does not produce
a stable output format (older binutils versions do not include
symbol version information).  This commit re-uses scripts/abilist.awk
with a tweak to include GLIBC_PRIVATE symbols.  This awk script
is based on objdump -T output, which appears to be stable over time.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>