aarch64 (and riscv64) don't have the deprecated sbrk.
I reported the issue upstream and I hope have time to remove sbrk soon.
At the moment I prefer to add this workaround, instead of mark this port as BROKEN, so we can take advantage of grass7's other algorithms on aarch64.
If we must mark this port BROKEN, we will have to add a condition to exclude grass7 from graphics/qgis and graphics/qgis-ltr to allow them to be built on aarch64 (hopefully no other dependencies will be broken)
Reported by mikael. Thanks!