While experimenting with switching the build to -std=c11 (https://mail-index.netbsd.org/tech-kern/2025/01/05/msg029919.html), I saw warnings fly by like this:
/home/riastradh/netbsd/current/obj.riscv/tooldir/bin/../lib/gcc/riscv64--netbsd/12.4.0/../../../../riscv64--netbsd/bin/ld: /home/riastradh/netbsd/current/src/../obj.riscv/destdir.riscv/lib/libc.so: warning: Warning: reference to the libc supplied alloca(3); this most likely will not work. Please use the compiler provided version of alloca(3), by supplying the appropriate compiler flags (e.g. -std=gnu99).