"Resource temporarily unavailable" when creating threads

[nia <nia%NetBSD.org@localhost>]

From a recent aarch64 bulk package build report:

firefox fails with:

thread 'rustc' panicked at 'failed to spawn helper thread: Os { code: 35, kind: WouldBlock, message: "Resource temporarily unavailable" }', /pbulk/work/lang/rust/work/rustc-1.43.1-src/src/librustc_codegen_ssa/back/write.rs:1000:18

erlang fails with:

Failed to create thread: Resource temporarily unavailable (35)
make[4]: *** [aarch64-unknown-netbsd9.0/Makefile:743: obj/aarch64-unknown-netbsd9.0/opt/smp/PROFILE] Abort trap (core dumped)

http://victory.netbsd.org/pkgsrc/packages/reports/HEAD/evbarm64-9.0/20200627.2244/firefox-77.0.1nb3/build.log
http://victory.netbsd.org/pkgsrc/packages/reports/HEAD/evbarm64-9.0/20200627.2244/erlang-22.2.8/build.log

is this a known bug? is it fixed in -stable?