Build with the "read-only relocation" hardening linker flag

Summary: Improve binary security by enabling a hardening linker flag.

Test Plan:
Patch already used on Debian.

Tested locally by @Gallaecio as well.

Reviewed By: LudovicRousseau

Trac Tickets: #4894

Differential Revision: https://code.wildfiregames.com/D1123