Support for Full RELRO

[Blacksmoke16]

I was messing around with adding a package to AUR and noticed namcap was producing a warning: ELF file ('usr/bin/oq') lacks FULL RELRO, check LDFLAGS, which apparently relates to an ELF hardening feature:

• https://www.redhat.com/en/blog/hardening-elf-binaries-using-relocation-read-only-relro
• https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/exploit-mitigations.md#read-only-relocations-and-immediate-binding
• https://web.archive.org/web/20200308175234/http://tk-blog.blogspot.com/2009/02/relro-not-so-well-known-memory.html

It seems this feature is enabled by default in Rust since Oct 2017. Probably wouldn't hurt to do the same in Crystal land? Could reference this PR: rust-lang/rust#43170.