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)