build: do not disable hardening on AArch64 systems

It was disabled because ligomp.a on some AArch64 systems (e.g. CentOS)
is compiled without PIC support. Now Tarantool doesn't depend on ligomp,
so it's safe to turn on PIC for AArch64 systems.

Follow-up #7536

NO_DOC=build
NO_TEST=build

(cherry picked from commit 5c1968ca)