Rollup merge of #156638 - FelixLttks:fix-espidf-sigkill, r=SimonSapin

library: Fix std compilation for espidf target in unix::process

Fixes a regression on the riscv32imac-esp-espidf target caused by commit 7bf5fe7bf84f5b94e5970de18543abac0579209c (linked issue rust-lang/rust#156537) . The unix_kill_process_group feature attempts to use libc::SIGKILL, which is not supported on the espidf target.

Discussed in `esp-idf-sys` issue: https://github.com/esp-rs/esp-idf-sys/issues/419