This fixes building on Bullseye.
According to https://github.com/libretro/dosbox-pure/commit/603b1c7ae
>Switch to gcc 9 to avoid buggy assembly genetation [sic] of gcc 10
>On armv7l, gcc 10.2 with -O2 on the file core_dynrec.cpp generates assembly that wrongfully passes NULL to the runcode function
>resulting in a segfault crash. It can be observed by writing block->cache.start to stdout twice where it is NULL at first
>and then the actual value thereafter. This affects upstream SVN DOSBox as well as this core.
