build: do not disable hardening on FreeBSD

It was disabled because dlsym tests failed when PIC is on (#7640).
However, later it turned out that the issue is not related to PIC, and
LuaJIT was turned off for such tests by commit 67e79b15 ("test: turn
LuaJIT off in tests with dlsym"). Now it's safe to turn on PIC for FreeBSD.

Follow-up #7536

NO_DOC=build
NO_TEST=build

(cherry picked from commit 05eba830)

changelogs/unreleased/gh-7536-enable-hardening-on-FreeBSD.md

+## feature/build
+
+* Hardening against memory corruption attacks is now enabled by default on
+  FreeBSD (gh-7536).

cmake/hardening.cmake

 # Depends on os.cmake and profile.cmake modules.
-# Uses `ENABLE_FUZZER` option and `TARGET_OS_FREEBSD` variable.
+# Uses `ENABLE_FUZZER` option and `TARGET_OS_DARWIN` variable.
 
-# LuaJIT in FreeBSD doesn't work with PIC (gh-7640),
 # ligomp.a for AArch64 CentOS is compiled without PIC support.
-if (ENABLE_FUZZER OR TARGET_OS_FREEBSD OR ${CMAKE_SYSTEM_PROCESSOR} MATCHES "aarch64")
+if (ENABLE_FUZZER OR ${CMAKE_SYSTEM_PROCESSOR} MATCHES "aarch64")
     set(ENABLE_HARDENING_DEFAULT FALSE)
 else()
     set(ENABLE_HARDENING_DEFAULT TRUE)