gdb: update mdebugread.c to use blockvector::block_less_than

This commit updates mdebugread.c to use common blockvector ordering
predicate. It also changes the code to use std::stable_sort as in
buildsym.c. This is probably not necessary but should not hurt and makes
block sorting code more consistent.

Approved-By: Tom Tromey <tom@tromey.com>