[Flang-RT][OpenMP] Define _GLIBCXX_NO_ASSERTIONS (#155440)
Since GCC 15.1, libstdc++ enabled assertions/hardening by default in
non-optimized (-O0) builds [1]. That is, _GLIBCXX_ASSERTIONS is defined
in the libstdc++ headers itself so defining/undefining it on the
compiler command line no longer has an effect in non-optimized builds.
As the commit message[2] suggests, define _GLIBCXX_NO_ASSERTIONS
instead.
For libstdc++ headers before 15.1, -U_GLIBCXX_ASSERTIONS still has to be
on the command line as well.
Defining _GLIBCXX_NO_ASSERTIONS was previously proposed in #152223
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112808
[2] https://github.com/gcc-mirror/gcc/commit/361d230fd7800a7e749aba8ed020f54f5c26d504
