textproc/difftastic: fix build on powerpc

= note: ld: error: undefined symbol: __atomic_fetch_add_8
>>> referenced by static.c
>>>               static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
>>> referenced by static.c
>>>               static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
>>> referenced by static.c
>>>               static.o:(mi_stats_merge_from) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
>>> referenced 75 more times
ld: error: undefined symbol: __atomic_load_8
>>> referenced by static.c
>>>               static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
>>> referenced by static.c
>>>               static.o:(_mi_stats_print) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
>>> referenced by static.c
>>>               static.o:(_mi_stats_print) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
ld: error: undefined symbol: __atomic_compare_exchange_8
>>> referenced by static.c
>>>               static.o:(mi_stat_update) in archive /wrkdirs/usr/ports/textproc/difftastic/work/rustc5gqLZw/liblibmimalloc_sys-3697f752fef9d30f.rlib
cc: error: linker command failed with exit code 1 (use -v to see invocation)