Configure GCC with --enable-default-pie

[MaskRay]

Due to ASLR security hardening, most Linux distributions have switched to default PIE for their GCC builds for many years. Actually I cannot find a Linux distribution which doesn't use default PIE. See https://gist.github.com/thesamesam/f7a9c17df0165e86babb189fda8d3048 for a survey.

Many GCC build scripts like https://github.com/richfelker/musl-cross-make , glibc scripts/build-many-glibcs.py specify --enable-default-pie as well.

At this point, I think it makes more sense for compiler-explorer to match the Linux status quo, rather than the upstream default.

GCC feature request about switching the default for Linux: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103398