default.conf: enable _GLIBCXX_ASSERTIONS

these have a slight runtime hit (like fortify-source), but help find
bugs early, by making programs crash on invariants that would corrupt
memory and lead to hard to debug crashes/bugs later.

default.conf

 export CFLAGS="-Os -Wformat -Werror=format-security"
-export CXXFLAGS="-Os -Wformat -Werror=format-security"
+export CXXFLAGS="-Os -Wformat -Werror=format-security -D_GLIBCXX_ASSERTIONS=1"
 export LDFLAGS="-Wl,--as-needed,-O1,--sort-common"
 export GOFLAGS="-buildmode=pie -modcacherw -trimpath -buildvcs=false"
 # Do note that these should work with at least GDC and LDC