meson: Disable GCC's dead store elimination for memory zeroing custom new
Some classes use custom new operator which zeroes memory, however gcc does aggressive dead-store elimination which threats all writes to the memory before the constructor as "dead stores".
For now we disable this optimization.
The new operators in question are declared via:
The issue was found with lto builds, however there is no guarantee that it didn't happen with ordinary ones.
I think we should properly initialize all fields in constructors, however it's safer to disable optimization for 20.0.8 and for 20.1.0 releases.