nptl_db/Makefile: force C locale when running readelf

db-symbols.awk relies on English-language readelf output to find the
beginning of the symbol table, so force the locale.

Without this change, "make check" fails in non-English locales:

| diff -p -U 0 ../sysdeps/unix/sysv/linux/x86_64/64/nptl/libthread_db.abilist [...]/nptl_db/libthread_db.symlist
| readelf -W -s [...]/nptl/libpthread.so | gawk -f [...]/nptl_db/db-symbols.v.i > [...]/nptl_db/db-symbols.out
| make[2]: *** [[...]/nptl_db/db-symbols.out] Erreur 1