Fix ASan build on FreeBSD

[vkrivopalov]

This MR fixes LuaJIT ASan-instrumented builds on FreeBSD, which currently fail with:

CMake Error at test/cmake/LibRealPath.cmake:27 (message):
Library 'libstdc++.so' is not found

Two issues combine to cause this failure:

1. add_subdirectory(test) is called unconditionally in the root CMakeLists.txt, even when LUAJIT_USE_TEST=OFF.
   This means test configuration runs regardless of whether tests are needed.
2. When LUAJIT_USE_ASAN=ON, test/LuaJIT-tests/CMakeLists.txt searches for libstdc++.so to set up an LD_PRELOAD workaround for AddressSanitizer CHECK failed: ../../../sanitizer/asan/asan_interceptors.cc:384 "((__interception::real___cxa_throw)) when using --as-needed and dynamically loaded .so google/sanitizers#934. This workaround is GCC-specific, but FreeBSD uses clang with libc++, not libstdc++.

Verified on FreeBSD 15.0 with clang 19:

mkdir build && cd build
cmake .. -DLUAJIT_USE_ASAN=ON
make -j$(sysctl -n hw.ncpu)
ASAN_OPTIONS=verbosity=1 ./src/luajit -v 2>&1 | grep -i AddressSanitizer

[vkrivopalov]

Hi @Buristan,

I looked at another PR currently opened in this repo and found you reply that patches for Tarantool's LuaJIT should go into the mailing list.
Does this still apply? Should I send the patch there or can it be accepted here on GitHub?

[Buristan]

Hi, Vladimir!
Thanks for the patch!
Yes, it will be more convenient to proceed with our mailing lists.

Patch LGTM, but I ask for a minor fixup: to separate it into 2 commits:
1 -- for the test logic fix and the second one for the FreeBSD + ASAN fix.

You may then send the patchset to the mailing lists like the following:

git format-patch --cover-letter --thread=shallow --subject-prefix="PATCH luajit" HEAD~2
# Update 0000-cover-letter.patch header and description.
git send-email --cc=tarantool-patches@dev.tarantool.org --to="Sergey Kaplun <skaplun@tarantool.org>" --to="Sergey Bronnikov <sergeyb@tarantool.org>" 000*

Before using git send-email command, you need to configure it.
If you use a GMail account, add the following code to your .gitconfig:

	[sendemail]
	    smtpencryption = tls
	    smtpserver = smtp.gmail.com
	    smtpserverport = 587
	    smtpuser = your.name@gmail.com
	    smtppass = topsecret

For mail.ru (or yandex.ru with the corresponding changes) users, the configuration will be slightly different:

	[sendemail]
	    smtpencryption = ssl
	    smtpserver = smtp.mail.ru
	    smtpserverport = 465
	    smtpuser = your.name@mail.ru
	    smtppass = topsecret